Nitride-composite semiconductor laser element, its manufacturing method, and semiconductor optical device

ABSTRACT

A nitride semiconductor laser device with a reduction in internal crystal defects and an alleviation in stress, and a semiconductor optical apparatus comprising this nitride semiconductor laser device. First, a growth suppressing film against GaN crystal growth is formed on the surface of an n-type GaN substrate equipped with alternate stripes of dislocation concentrated regions showing a high density of crystal defects and low-dislocation regions so as to coat the dislocation concentrate regions. Next, the n-type GaN substrate coated with the growth suppressing film is overlaid with a nitride semiconductor layer by the epitaxial growth of GaN crystals. Further, the growth suppressing film is removed to adjust the lateral distance between a laser waveguide region and the closest dislocation concentrated region to 40 μm or more.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/211,577 filed Sep. 16, 2008, now U.S. Pat. No. ______, which is adivisional of U.S. patent application Ser. No. 10/493,137 filed Oct. 18,2004, now U.S. Pat. No. 7,498,608, which is a U.S. national stageapplication of an International Application No. PCT/JP02/11186 filed onOct. 28, 2002, which claims priority from Japanese Patent ApplicationNos. 2001-330068 and 2001-330181 filed Oct. 29, 2001, the contents ofwhich are incorporated herein by reference in their entireties.

This application is a divisional of application Ser. No. 10/493,137filed Oct. 18, 2004, which is the national stage filing ofPCT/JP02/11186 filed on Oct. 28, 2002, which claims priority to JapaneseApplication No. 2001-330068 filed on Oct. 29, 2001, in its entirety.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a nitride-composite semiconductor laserelement, which is constituted with plural nitride compositesemiconductor layers laminated on a nitride-composite semiconductorsubstrate, and also relates to a method of manufacturing it, while thepresent invention further relates to a semiconductor optical deviceusing the nitride-composite semiconductor laser element in subject.

Background Art

Conventionally, from the viewpoint of wave-band constitution andchemical stability, in the field of the III-V family nitridesemiconductors consisting of the III family elements such as aluminum(Al), gallium (Ga), and indium (In), and of nitrogen (N) correspondingto one of the V family elements (this will merely be referred to as aGaN composite semiconductor hereinafter), there has been muchexpectation on the potential utility of this GaN composite semiconductorfor its practical application to a light emitting element and a powerdevice or the like. In order to realize the potential utility of thissemiconductor, much effort has thus been pursued so as to manufacture anitride composite semiconductor laser element capable of emitting bluelaser beam, for example, by way of laminating the GaN compositesemiconductor layers on a sapphire substrate (composed of Al₂O₃) or on aSiC composite substrate.

Nevertheless, when a thin film consisting of a crystalline GaN compositeelement has been formed on a sapphire substrate or on a SiC compositesubstrate, due to difference in the lattice constant between the GaNcomposite element and the substrate material, various defectivephenomena such as “dislocation” for example are induced into anepitaxial layer. Because of this reason, in the case of a nitridecomposite semiconductor laser element enabling high-density current toflow through it, the above-cited defect triggers so as to cause thelattice constitution to become disordered, thereby further causing theabove defective phenomena to potentially propagate themselves. Further,when the nitride-composite semiconductor laser element has been formedon a sapphire substrate, there is a problem in terms of its practicalservice life. It is conceived that, this is probably because occurrenceof the high-density dislocation in the above element adversely acts,thereby restricting potential service life of the nitride-compositesemiconductor laser element.

Due to the above reason, ideally, any substrate usable for loading theabove GaN composite semiconductor device should preferably be composedof mono-crystalline GaN composite material. This constitution eliminatesthe difference of the lattice constant between the substrate andindividual layers laminated on the substrate. Further, because the GaNcomposite compound itself contains compatibility with cleavages, aprocess for cutting a wafer into a plurality of chips can befacilitated. Further, since the GaN composite crystals are electricallyconductive, processes for disposing electrodes in the above device canbe simplified. Owing to these advantages, mono-crystalline GaN compositematerial is conceivably most suitable for constituting a substrate.

There is a report on the result of development in the field of a nitridecomposite semiconductor laser element capable of oscillating itself in arange from the ultra-violet region up to the visible rays region asshown in “Jpn. J. Appl. Phys. Vol. 39 (2000)”, on pages L647 to 650. Thenitride composite semiconductor in subject features a constitution inwhich a SiO₂ mask pattern with periodically striped openings was formedon a GaN composite substrate, and further, a structure consisting oflaminated nitride semiconductor layers with a striped wave-guide passage(i.e., a ridge stripe structure) was formed on the above-cited maskpattern.

The above-cited GaN composite substrate was manufactured via executionof a process described below. By applying a MOCVD (Metalorganic ChemicalVapor-phase Deposition) method, a GaN composite layer with 15 μm ofthickness was formed on the grounding seed crystals superficially beingprovided with a SiO₂ mask pattern containing striped openings per 20 μmof cyclic period before eventually generating a wafer with a flatsurface. This method is conventionally defined as the ELOG (EpitaxiallyLateral Overgrown) method, which, by way of availing lateral growth,causes defective phenomena to be decreased. Further, by applying aconventional HVPE (Hydride Vapor Phase Epitaxy) method, a GaN compositelayer with 200 μm of thickness was formed, and then the grounding seedcrystals were removed before eventually completing the processes formanufacturing a GaN composite substrate. In terms of the service lifecharacteristics of the produced semiconductor laser element, it was soestimated that a maximum of 15000 consecutive service hours could beavailable under 30 mW of an output condition at 60° C. of environmentaltemperature.

Nevertheless, in the case of the above-cited nitride semiconductor laserelement, since the method for manufacturing the GaN composite substratenecessitated growth of crystals by three cycles including theabove-referred HVPE method, the MOCVD method for growing the groundingseed crystals, and the other MOCVD method for growing structure ofsemiconductor laser element, the method of manufacturing the above-citednitride composite semiconductor laser element involves much complexity,thus generating a problem in terms of the productive efficiency.Further, the service life characteristics were evaluated to be still notsufficient. In particular, the service life characteristics were stillinsufficient under a high output condition (at 70° C. and 60 mW forexample). In addition, it was found that crack could appear on thesurface of the film grown after the growth of the laminate structure,thereby potentially causing the yield to be lowered during theproduction stage.

Generation of the above problems is caused by the crystalline defect,i.e., the “dislocation” generated in the above nitride compositesemiconductor laser element. It was confirmed that, normally, thecrystalline defect was generated on the surface of the GaN compositesubstrate by approximately 5×10⁷cm⁻². By applying any effective meansfor bending or extinguishing the crystalline defect, it is possible togenerate such a region containing crystalline defect with a low density,thereby enabling to secure a sufficient service life under a high-outputcondition in which technical problems still remain without being solved.It is further suggested that, by providing a mechanism capable ofstructurally relaxing strain inside of crystal layers formed on the GaNcomposite substrate, it is possible to lower the probability of causingcrack to be generated, thereby preventing the yield rate from beinglowered otherwise caused by the crack generated on the surface of thegrown film.

DISCLOSURE OF THE INVENTION

In the light of the above problems, the present invention aims atproviding a nitride composite semiconductor laser element internallycontaining minimized crystalline defect and relaxed stress and alsoproviding a semiconductor optical device incorporating such a nitridecomposite semiconductor laser element.

To achieve the above object, according to one aspect of the presentinvention, in a nitride semiconductor laser device provided with anitride semiconductor substrate and a nitride semiconductor layer formedon top of the nitride semiconductor substrate, the nitride semiconductorsubstrate has, as a portion thereof, a stripe-shapeddislocation-concentrated region in which crystal defects concentrate andhas, elsewhere, a low-dislocation region, a growth-inhibiting film forinhibiting growth of a nitride semiconductor crystal is formed on asurface of the nitride semiconductor substrate in a position where thegrowth-inhibiting film covers the dislocation-concentrated region, andthe nitride semiconductor layer is formed by growing the nitridesemiconductor crystal on top of the nitride semiconductor substrate inthe position where the growth-inhibiting film is formed.

By virtue of the above arrangement, in the course of laminating thenitride composite semiconductor layers on the surface of the nitridecomposite semiconductor substrate, it is possible to inhibit dislocationof crystalline defect from those regions containing concentratedcrystalline defect from further spreading itself, thus making itpossible to prevent high-density crystalline defect portions fromspreading themselves throughout the entire nitride compositesemiconductor layers by proper effect of the growth inhibiting films.Accordingly, it is possible to lower the density of crystalline defectinside of the nitride composite semiconductor layers.

When constituting the above nitride composite semiconductor laserelement, it is also practicable to provide each of those regionsconcentrated with crystalline defect with a plurality of growthinhibiting linear films so that these linear films can be disposed inthe drain-board form, and further, it is also practicable to arrangethat each of those regions concentrated with the dislocated crystallinedefect can fully be covered with a plurality of growth inhibiting films.By virtue of this arrangement, those nitride composite semiconductorcrystals grown from those discrete regions containing low-densitydislocation can easily be combined with each other. Accordingly, unlikethe case of laminating nitride composite semiconductor layers as of thecondition in which nitride composite semiconductor crystals (grown fromthose regions each containing low-density crystalline defect) are notcombined with a sheet-form growth inhibiting film even after formationof this sheet-form growth inhibiting film thereon, cleavage formingprocess can readily be executed.

When implementing the above process for forming a plurality of growthinhibiting linear films on each of the regions concentrated with thedislocated crystalline defect, it is so arranged that each of the linearfilms will be provided with a minimum of 1 μm and a maximum of 10 μm ofwidth so as to be disposed in parallel with each other across a minimumof 1 μm and a maximum of 10 μm of intervals against individuallyadjoining growth inhibiting linear films, and further, it is so arrangedthat individual regions including a sum of the width and intervals of aplurality of these growth inhibiting films can fully cover thoseindividual regions concentrated with the dislocated crystalline defect.

Further, it is so arranged that the nitride composite semiconductorsubstrate can be provided with n-type conductive characteristics, andyet, in order that all the growth inhibiting linear films can becovered, a GaN composite film containing the n-type conductivecharacteristics is formed on the surface of the nitride compositesemiconductor substrate, thus completing formation of a GaN compositefilm with a plane surface. Owing to this arrangement, it is possible toprevent the high-density crystalline defect regions from propagatingthemselves throughout the entire nitride composite semiconductor layers.Further, by providing the nitride composite semiconductor substrate withthe n-type conductivity having a high resistance value, it is possibleto serially laminate each of the nitride composite semiconductor layersaccording to the sequence of the n-type followed by the p-type. This inturn contributes to an improvement of the superficial flatness of thenitride composite semiconductor layers formed with grown crystals,thereby enabling to decrease the threshold value of current required forthe output of laser beam. In the course of forming the GaN compositefilm containing the n-type conductive characteristics, the filmthickness is arranged to be a minimum of 1 μm and a maximum of 20 μm.

Further, by way of arranging the thickness of each of the growthinhibiting linear films to be a minimum of 0.05 μm and a maximum of 1μm, it is possible to enable the growth inhibiting films to individuallyexert own proper effect and also prevent the growth inhibiting filmsfrom generating adverse influence. In the present invention, it isdefined that the above-referred growth inhibiting film consists of asilicon compound film or a metallic film. In particular, in this case,the above referred growth inhibiting film shall consist of a thin filmmade from SiO₂, Si₃N, Ti (titanium), or W (tungsten).

In the course of manufacturing the above nitride composite semiconductorlaser element, it may be so arranged that the nitride compositesemiconductor layers further contain quantum well active layer having acomposition expressed in terms of “In_(X)Ga_(1-X)N (0<x<1)”. Further, itmay be so arranged that at least any of those elements including As(arsenic), P (phosphor), and Sb (antimony), shall be contained in theactive layer. In particular, it is preferred that the above nitridecomposite semiconductor substrate be composed of the GaN compositeelements.

According to another aspect of the present invention, a method offabricating a nitride semiconductor laser device including a nitridesemiconductor substrate and a nitride semiconductor layer formed on topof the nitride semiconductor substrate includes the steps of forming, ona surface of the nitride semiconductor substrate, which has, as aportion thereof, a stripe-shaped dislocation-concentrated region inwhich crystal defects concentrate and has, elsewhere, a low-dislocationregion, a growth-inhibiting film for inhibiting growth of a nitridesemiconductor crystal in a position where the growth-inhibiting filmcovers the dislocation-concentrated region; and then forming the nitridesemiconductor layer by growing the nitride semiconductor crystal on topof the nitride semiconductor substrate in the position where thegrowth-inhibiting film is formed.

It is also practicable to initially form the growth inhibiting films allover the surface of the above nitride composite semiconductor substratefollowed by execution of an etching process so as to cover only thoseregions concentrated with dislocated crystalline defect with the growthinhibiting films. Further, availing of the n-type electrical conductivecharacteristics of the nitride composite semiconductor substrate, it isalso practicable to form a GaN composite film incorporating the n-typeconductive characteristics on the nitride composite semiconductorsubstrate so as to fully conceal the growth inhibiting films aftercovering those regions concentrated with dislocated crystalline defectwith the growth inhibiting films and then followed by execution of aprocess for causing the nitride composite semiconductor crystals to begrown on the surface of the formed GaN composite film before eventuallylaminating the nitride composite semiconductor layers thereon.

According to another aspect of the present invention, in a nitridesemiconductor laser device provided with a nitride semiconductorsubstrate and a nitride semiconductor layer formed on top of the nitridesemiconductor substrate, the nitride semiconductor substrate has, as aportion thereof, a stripe-shaped dislocation-concentrated region andhas, elsewhere, a low-dislocation region, the nitride semiconductorlayer has a stripe-shaped laser light guide region, the laser lightguide region is located above the low-dislocation region andsubstantially parallel to the dislocation-concentrated region, and adistance d in a horizontal direction between the laser light guideregion and a dislocation-concentrated region closest thereto is 40 μm ormore. Displacing the laser beam guide region from the regionsconcentrated with dislocated crystalline defect by more than 40 μmprevents adverse influence potentially caused by the dislocation of thesubstrate from affecting the laser beam guide regions, thereby making itpossible to secure a durable semiconductor laser element capable ofextending its service life in terms of the laser oscillating capability.

According to another aspect of the present invention, in a nitridesemiconductor laser device provided with a nitride semiconductorsubstrate and a nitride semiconductor layer formed on top of the nitridesemiconductor substrate, the nitride semiconductor substrate has, asportions thereof, a plurality of dislocation-concentrated regions in ashape of stripes substantially parallel to one another and has,elsewhere, low-dislocation regions, the nitride semiconductor layer hasa stripe-shaped laser light guide region, the laser light guide regionis located above the low-dislocation regions and substantially parallelto the dislocation-concentrated regions, a distance d in a horizontaldirection between the laser light guide region and adislocation-concentrated region closest thereto is 40 μm or more, and,assuming that a middle region of each low-dislocation region is locatedalong a middle line between adjacent dislocation-concentrated regions, adistance t in the horizontal direction between the laser light guideregion and a middle region, closest thereto, of a low-dislocation regionis 30 μm or more. Presence of a plurality of regions concentrated withdislocated crystalline defect in a nitride semiconductor substrate maycreate, in a central portion of a region containing crystalline defectwith a low density, a region having slightly different properties fromthe region surrounding it. By displacing the laser beam guide region by30 μm or more from such a region containing crystalline defect with alow density, it is possible to obtain a semiconductor laser element witha long single-layer laser oscillation life.

According to another aspect of the present invention, in a nitridesemiconductor laser device provided with a nitride semiconductorsubstrate and a nitride semiconductor layer formed on top of the nitridesemiconductor substrate, the nitride semiconductor substrate has, as aportion thereof, a stripe-shaped dislocation-concentrated region andhas, elsewhere, a low-dislocation region, the low-dislocation regionhave a high-luminescence region, the dislocation-concentrated region issubstantially parallel to the high-luminescence region, the nitridesemiconductor layer has a stripe-shaped laser light guide region, thelaser light guide region is located above the low-dislocation region andsubstantially parallel to the dislocation-concentrated region, adistance d in a horizontal direction between the laser light guideregion and a dislocation-concentrated region closest thereto is 40 μm ormore, and a distance t in the horizontal direction between the laserlight guide region and a high-luminescence region closest thereto is 30μm or more. By way of displacing the laser beam guide region from one ofthe high luminescence regions having specific characteristics differentfrom that of the peripheral regions, it is also possible to produce asemiconductor laser element incorporating an extensible service life.

Desirably, the distance “p” between adjoining regions each beingconcentrated with stripe-form dislocated crystalline defect presentinside of the nitride composite semiconductor substrate shall be withina minimum of 140 μm. Provision of this distance “p” facilitates theprocess for displacing the laser beam guide region from those regionsconcentrated with dislocated crystalline defect during the productionstage.

It is further desired that the distance “p” shall remain within amaximum of 1000 μm in order that any unwanted portion displaced from thebottom portion of the laser beam guide region among those discreteregions each accommodating low-density dislocation will not be able tofurther spread excessively.

It is possible to use such a nitride composite semiconductor substrateinternally containing those regions concentrated with stripe-formdislocated crystalline defect being disposed substantially in parallelwith the direction [1-100] of the substrate itself.

According to another aspect of the present invention, in a nitridesemiconductor laser device provided with a nitride semiconductorsubstrate and a nitride semiconductor layer formed on top of the nitridesemiconductor substrate, the nitride semiconductor substrate has astripe-shaped high-luminescence region, the nitride semiconductor layerhas a stripe-shaped laser light guide region, the laser light guideregion is substantially parallel to the high-luminescence region, and adistance t between the laser light guide region and a high-luminescenceregion closest thereto is 30 μm or more. By way of displacing the laserbeam guide region from one of the high luminescence regions havingspecific characteristic differing from that of the peripheral highluminescence regions, it is possible to produce a durable semiconductorlaser element with an extensible service life.

It is possible to use such a nitride-composite semiconductor substrateincorporating a certain number of high luminescence regions in its[1-100] direction.

It is also possible to constitute the nitride-composite semiconductorlayers incorporating quantum well active layer having a compositiondefined as “In_(x)Ga_(1-x)N (0<x<1)”.

Further, it is also possible to constitute the nitride compositesemiconductor incorporating quantum well active layer consisting ofnitride composite semiconductor containing at least any of thoseelements including arsenic (As), phosphor (P), and antimony (Sb).

According to another aspect of the present invention, a method offabricating a nitride semiconductor laser device including the step offorming, on a nitride semiconductor substrate having, as portionsthereof, a plurality of dislocation-concentrated regions in a shape ofstripes substantially parallel to one another and has, elsewhere,low-dislocation regions, a nitride semiconductor layer including anitride semiconductor laser structure having a stripe-shaped laser lightguide region further includes the steps of forming the laser light guideregion above a low-dislocation region; and leaving a distance d of 40 μmor more in a horizontal direction between the laser light guide regionand a low-dislocation region closest thereto. When implementing thismethod, it is so arranged that the laser beam guide region is disposedabove the discrete regions accommodating low-density dislocationsubstantially in parallel with those regions concentrated withdislocated crystalline defect, and further it is so arranged that thehorizontal directional distance “d” between those laser beam guideregions and those regions concentrated with dislocated crystallinedefect being closest to the laser beam guide region to be a minimum of40 μm. The above arrangement prevents adverse influence potentiallycaused by the dislocation of the substrate from affecting the laser beamguide regions, thereby making it possible to secure a durablesemiconductor laser element capable of extending its service life interms of the laser oscillating capability.

When the center line portion of the adjoining striped regionsconcentrated with dislocated crystalline defect corresponds to thecenter line portion of those regions containing low-density arrangement,it is also possible to determine the horizontal directional distance “t”between the laser beam guide region and those regions containinglow-density dislocation to be a minimum of 30 μm. By displacing thelaser beam guide regions from the center portion of a region containinglow-density dislocation, which may have specific characteristicsslightly differing from that of the peripheral regions, it is alsopossible to secure a durable semiconductor laser element with anextensible service life.

It is suggested that, when using a nitride-composite semiconductorsubstrate containing a striped high-luminescence region substantially inparallel with those regions concentrated with stripe-form dislocatedcrystalline defect within those regions accommodating low-densitydislocation, the horizontal directional distance “t” between the laserbeam guide region and one of the high-luminescence regions being closestto this laser beam guide region may be arranged to be a minimum of 30μm. By way of displacing the laser beam guide region from one of thehigh-luminescence regions containing specific characteristics slightlydiffering from that of the peripheral regions, it is possible to securea semiconductor laser element with a further extensible service life.

It is recommended to use such a nitride composite semiconductorsubstrate internally containing those adjoining regions concentratedwith dislocated crystalline defect across a minimum of 140 μm of thedistance “p” between them. This arrangement facilitates displacement ofthe laser beam guide regions from those regions concentrated withstripe-form dislocated crystalline defect.

In addition, it is also recommended to use a nitride compositesemiconductor substrate internally containing those adjoining regionsconcentrated with dislocated crystalline defect across a maximum of 1000μm of the distance “p”. This arrangement avoids generation of such adefective semiconductor laser element containing much unwanted portionsnot normally being present below the laser beam guide regions amongthose regions internally containing low-density dislocation, and yet,this arrangement further improves the yield rate.

According to another aspect of the present invention, a method offabricating a nitride semiconductor laser device including the step offorming, on a nitride semiconductor substrate having a plurality ofstripe-shaped high-luminescence regions substantially parallel to oneanother, a nitride semiconductor layer including a nitride semiconductorlaser structure having a stripe-shaped laser light guide region furtherincludes the steps of forming the laser light guide region substantiallyparallel to the high-luminescence regions; and leaving a distance t of30 μm or more in a horizontal direction between the laser light guideregion and a high-luminescence region closest thereto. This method alsomakes it possible to secure a durable semiconductor laser element withan extensible service life.

It is possible to use such a nitride composite semiconductor substratecontaining those regions concentrated with dislocated crystallinedefect, wherein the substrate has saw-teethed projections and recessessuperficially provided with facets {11-22} and accommodates thoseregions concentrated with dislocated crystalline defect below the bottomof the superficial projections and recesses.

It is further possible to use such a nitride composite semiconductorsubstrate containing a certain number of high-luminescence regions,wherein the substrate has saw-teethed projections and recessessuperficially provided with facets {11-22} and accommodates thehigh-luminescence regions beneath the vertex of the superficialprojections and recesses.

The semiconductor optical device according to the present inventionfeatures that the above-described nitride composite semiconductor laserelement constitutes a light source.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A to FIG. 1D are schematically explanatory of serial processes formanufacturing an n-type GaN composite semiconductor substrate;

FIG. 2 is a cross-sectional view of the internal constitution of anitride-composite semiconductor laser element according to a firstembodiment of the present invention;

FIG. 3 is a cross-sectional view representing an aspect of the nitridecomposite semiconductor layers laminated on the n-type GaN compositesemiconductor substrate according to the first embodiment of the presentinvention;

FIG. 4 is a cross-sectional view representing another aspect of thenitride composite semiconductor layers laminated on an n-type GaNcomposite semiconductor substrate.

FIG. 5 is a cross-sectional view representing another aspect of thenitride composite semiconductor layers laminated on an n-type GaNcomposite semiconductor substrate;

FIG. 6A to FIG. 6C are respectively a cross-sectional view forexplanatory of serial steps for growing GaN composite crystals accordingto a second embodiment of the present invention;

FIG. 7 is a vertical cross-sectional view for schematically showing theconstitution of a semiconductor laser element according to a thirdembodiment of the present invention;

FIG. 8 is a vertical cross-sectional view for schematically showing thestratum constitution of the semiconductor laser element according to thethird embodiment of the present invention;

FIG. 9 is a chart for explanatory of the relationship between thedistance between the laser beam guide region and the region concentratedwith dislocated defective crystals and the laser oscillating life in thenitride composite semiconductor laser element related to the thirdembodiment of the present invention;

FIG. 10 is a chart for explanatory of the relationship between thedistance between the laser beam guide region and the high luminescenceregion and the laser oscillating life in the nitride compositesemiconductor laser element related to the third embodiment of thepresent invention;

FIG. 11 is a plan view for schematically showing a method ofmanufacturing a semiconductor laser element according to a fourthembodiment of the present invention;

FIG. 12 is a plan view for schematically showing a method ofmanufacturing a semiconductor laser element according to a fifthembodiment of the present invention;

FIG. 13 is a plan view for schematically showing a method ofmanufacturing a semiconductor laser element according to a sixthembodiment of the present invention;

FIG. 14 is a vertical cross-sectional view for schematically showingfurther stratum constitutions of a semiconductor laser element accordingto the third to sixth embodiments of the present invention; and

FIG. 15 is a simplified block diagram of the internal constitution of asemiconductor optical device according to the tenth embodiment of thepresent invention which incorporates the nitride composite semiconductorlaser element.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to the accompanying drawings, practical forms forimplementing the present invention are described below.

It should be understood that the nitride composite semiconductorsubstrate described in this specification at least consists of compositeelements defined as “Al_(x)Ga_(y)In_(z)N (0≦x≦1, 0≦y≦1, 0≦z≦1,x+y+z=1)”. It should also be understood that approximately less than 20%of the nitrogen component for constituting the inventive nitrogencomposite semiconductor substrate may be substituted with at least anyof those elements including arsenic (As), phosphor (P), and antimony(Sb).

When constituting the nitride composite semiconductor substrate relatedto the present invention, most desirably, binary crystalline GaNcomposite element is used. Provision of binary crystalline GaN compositeelement stabilizes the elementary composition, thereby making itpossible to readily secure stable characteristics suitable for asubstrate. Further, in the case of growing epitaxial layers on thesubstrate, there is no fear of causing the epitaxial composition toincur variation. Further, provision of the GaN composite elementgenerates satisfactory electrical conductivity. Next, the AlGaNcomposite substrate is described below. Inasmuch as the AlGaN compositesubstrate is constituted with composite elements having own index ofrefraction being less than that of the GaN composite element, whenconstituting a semiconductor laser component capable of emitting lightranging from UV region to blue region with the AlGaN composite element,it is possible to more effectively confine laser beam within its activelayer.

The nitride composite semiconductor substrate may be added withimpurities such as n-type or p-type dopant or the like. Availableimpurities include chlorine (Cl), oxygen (O), sulfur (S), selenium (Se),tellurium (Te), carbon (C), silicon (Si), germanium (Ge), zinc (Zn),cadmium (Cd), magnesium (Mg), and beryllium (Be). It is desired that atotal amount of addable impurities shall be a minimum of “5×10¹⁶ cm⁻³”and a maximum of “5×10²⁰ cm⁻³”. In order to provide the nitridecomposite semiconductor substrate with the n-type electricalconductivity, among those impurities cited above, any of those elementsincluding Si, Ge, O, Se, and Cl, is particularly suitable for use.

The constitution of the nitride composite semiconductor layers laminatedon the nitride composite semiconductor substrate is practically definedas “Al_(x)Ga_(y)In_(z)N (0≦x≦1, 0≦y≦1, 0≦z≦1, x+y+z=1)”. A certainamount corresponding to approximately less than 10% of the nitrogenconstituent for composing the nitride composite semiconductor layers maybe substituted with at least any of those elements including As, P, andSb on condition, as far as the semiconductor layers belong to hexagonalcrystal system.

Further, the nitride composite semiconductor layers may also be addedwith at least any of the above-cited impurities including Si, O, Cl, S,C, Ge, Zn, Cd, Mg, and Be. It is desired that a total amount of addableimpurities shall be a minimum of 5×10¹⁶ cm⁻³ and a maximum of 5×10²⁰cm⁻³ . In order to provide the nitride composite semiconductor layerswith the n-type electrical conductivity, among the above-cited addableimpurities, any of those elements including Si, Ge, S, Se, and Te, isparticularly suitable for use. For the sake of providing the p-typeelectrical conductivity, any of those elements including Mg, Cd, and Be,is particularly suitable for functioning as impurities addable to theabove layers.

It is defined that the term “active layer” implies the general terms ofa well layer or a composite well consisting of a well layer and abarrier layer. For example, an individual active layer consisting of amono-quantum well structure is composed of a single well layer or acomposite structure consisting of a combination of a barrier layer, awell layer, with another barrier layer. Another active layer based on amultiple quantum well structure consists of a plurality of well layersand a plurality of barrier layers.

The GaN composite element is hexagonal, and thus, in order to representaxial direction and surface direction, a specific notation using fourindices is introduced. Axis “a” and axis “b” jointly form 120 degreesand have an equal length (a=b), whereas axis “c” being orthogonal tothese axes “a” and “b” is a unique one without being equal to the axis“a” (c≠a). When the axes “a” and “b” are solely present, symmetry cannotbe represented along the direction of the “a”/“b” surface, and thus, anadditional axis is assumed to be axis “d”. It should be understood that,although directional designation can duly be effected by the axes “a”and “b”, in order that symmetry will not be lost, an additional axis “d”is introduced. Due to this reason, the axes “a”, “b”, and “d”, are notindependent of each other.

When it is assumed that a group of parallel surfaces is represented byfour indices (k, l, m, and n), this in turn means that the distance fromthe origin to the points at which the first surface counted from theorigin cuts the axes “a”, “b”, “d”, and “c” corresponds to a/k, b/l,d/m, and c/n. This definition is identical to the case of other crystalsystems. It should be understood however that, since the axes a, b, andd, constitute a redundant coordinate, those indices k, l, and m, are notindependent of each other, but instead, k+l+m=0 is stationary at anytime.

Definition in the case of dealing with the axis “c” is identical to thatis applicable to cubic system. Assume that n-units of surfaces inparallel with each other are present along the unit length of the axis“c”, an index in the direction of the direction “c” becomes “n”. Due tothe above reason, among four indices, each of the former three indiceshas own rotation symmetry except for the index of the axis “c” becausethe index of the axis “c” is independent of the others.

Individual surface direction is represented by (• • •), whereascollective surface direction is expressed by {• • •}. The term“collective” means assembly of all the surface directions that can bereached by way of performing all the symmetrical operations for enablinga certain crystal system to accept a specific surface direction.Crystalline direction is also represented by these indices. Whenexpressing the crystalline direction, an index corresponding to that ofa surface perpendicular to the crystalline direction is used. Individualdirection is represented by [• • •], whereas collective direction isrepresented by <• • •>. Although the above notations are commonly knownin the crystallography, in order to enable the concerned to betterunderstand the description, the above meaning has been explained.According to the rule of the crystallography, any negative index isdenoted by way of drawing a horizontal line right above thecorresponding numeral so as to enable a viewer to instinctively identifythe negative index. However, it is not practicable to draw a horizontalline right above the corresponding numeral in the present description,and thus, a sign “−” is shown just before the corresponding numeral sothat a negative number can be identified.

[Method of Manufacturing a GaN Composite Substrate]

First, referring to FIG. 1A to FIG. 1D, a method of manufacturing a GaNcomposite substrate superficially being formed with nitride compositesemiconductor layers required for eventually producing a nitridecomposite semiconductor laser element is described below. It should beunderstood that FIG. 1A to FIG. 1D respectively designate serialprocesses to be executed for manufacturing an n-type GaN compositesemiconductor substrate.

In the production of the n-type GaN composite substrate, slantedportions called “facets” are generated in the surface of growingcrystal. The term “facet” refers to a surface other than the surface(growth surface) perpendicular to the growth direction. By following upgrowth while maintaining the facet generation, it is possible to causethe “dislocation” to spread itself in the growing direction before beingassembled into a predetermined position. The region accommodating thefacets becomes the above-referred “region accommodating low-densitydislocation” due to migration of the crystalline defect, i.e., the“dislocation”. In addition, beneath the facets appear those regionscontaining high-density crystalline defect each being located across adefinite interface. Since the “dislocation” assembles at the interfaceor inside of the regions containing high-density crystalline defect(corresponding to the region concentrated with the “dislocated”crystalline defect to be described later on), the dislocated crystallinedefect disappears or builds up in the region containing high-densitycrystalline defect.

When the above condition is present, depending on the shape of thoseregions accommodating high-density crystalline defect, shape of thefacet varies. Concretely, when those regions containing high-densitycrystalline defect are dotted with each other, the facet is formed so asto surround those regions containing high-density crystalline defect,thereby generating pits consisting of the facet. On the other hand, whenthose regions containing high-density crystalline defect are formed inthe form of stripe pattern, facets are formed on both sides of each ofthose regions containing high-density crystalline defect so as to placethe striped portion of the region containing high-density crystallinedefect to become the bottom of a groove having a V-shaped cross-section.

In order to form those regions containing high-density crystallinedefect, it is essentially required to previously form seeds consistingof amorphous or polycrystalline layers required for generating the“dislocation” at predetermined portions for forming those regionscontaining high-density crystalline defect on a supporting substrateavailable for the grounding substrate. By causing GaN composite materialto be grown on the supporting substrate superficially forming the seedsfor generating the “dislocation”, those regions each containinghigh-density crystalline defect are formed in those portions right abovethe dislocation-forming seeds. Further, by way of causing the GaNcomposite layers to be grown in those regions each containinghigh-density crystalline defect, it is possible to proceed with aprocess for growing the GaN composite material while maintaining thefacets without necessarily burying it.

Concretely, when causing the n-type GaN layer 22 to be grown on thesupporting substrate 21 by applying the hydride vapor-phase epitaxialprocess (HVPE), it is so arranged that the facet {11-22} 23 can mainlybe exposed to the under-growth surface. In consequence, as shown in FIG.1A, cross-section of the surface portion becomes saw-teethed projectionsand recesses except for a local portion close to the vertex of each ofthe projections corresponding to the {0001} surface 26 slightly beingexposed as a stripe.

When implementing the above-referred HVPE process, initially, a Ga boatis set to the upstream side of a hot-wall type reaction furnace so as toblow HCl gas against melted Ga liquid, and a substrate 21 is disposed tothe downstream side of the hot-wall type reaction furnace so as to causethe GaN layer 22 to be grown via a blow of NH₃ gas against it. Next, theHCl gas is injected into the melted Ga so as to synthesize GaClcomposite element. The synthesized GaCl composite element is broughtdownwards, and then subject to a reaction with the NH₃ gas so as tosynthesize the GaN composite element, which is eventually deposited onthe substrate 21.

When implementing the HVPE process, a wafer consisting of GaAs compositeelement with 2 inches of diameter and having a surface (111) was used asthe substrate 21. The above-referred projections and recesses arerespectively provided with 400 μm of periodical pitches and extended inthe depth direction as seen in FIG. 1A. The GaAs wafer can easily beremoved after completing production of an ingot of an n-type GaN layer22 (to be described later on) subsequent to the growth of the GaNcomposite element, and thus, the GaAs wafer is more suitable forconstituting the substrate 21 than the case of applying a sapphirewafer. In order to regulate positions of the above projections andrecesses, it is recommended that, initially, SiO₂ masks each having acertain number of openings corresponding to the recesses be formed inadvance, and then crystalline growth be carried on as of the state inwhich the facets remain exposed. Note that the above openings correspondto the above-referred seeds required for forming the “dislocation”.

More specifically, the openings of the SiO₂ masks are respectivelydisposed in the stripe form across 400 μm of pitch so as to become inparallel with the direction [1-100] of the GaN composite crystals. It ispossible to arrange the form of the SiO₂ masks to be of continuousstripes or aligned in rows in the individually dotted formation. Thefollowing description refers to a practical example in the production ofthe GaN composite substrate by way of forming the striped SiO₂ masksacross 400 μm of intervals. However, the intervals between individualopenings may not necessarily be 400 μm. Desirably, the intervals betweenthem shall be a minimum of 100 μm, preferably, the intervals shall be aminimum of 200 μm and a maximum of 600 μm.

A method (a condition) for growing crystals as of the state of holdingthe facet {11-22} 23 externally being exposed was disclosed via theJapanese Laid-Open Patent Publication No. 2001-102307 under a previouspatent application filed by the inventor of the present invention. Bydoping oxygen on the way of growing crystals, polarity of the crystalsunder growth becomes the n-type.

As described above, by way of sustaining growth of crystals as of thestate in which the facets remain being exposed so as to further sustainformation of GaN composite crystals, as shown in FIG. 1B, an ingotconsisting of the n-type GaN composite layers with 30mm of height iseventually produced on the substrate 21. Simultaneously, facetscorresponding to the masks for the seeds are also formed on the ingotsurface. Concretely, when the masks are in the form of dotted patterns,all the pits consisting of the facets are regularly formed. On the otherhand, when the masks are in the form of stripe-like patterns, V-shapedfacets are formed.

Next, the ingot consisting of the n-type GaN composite layers 22 issliced into a plurality of thin pieces via a slicing tool so as tosecure the n-type GaN composite substrates. The sliced thin pieces arethen subject to a grinding process before eventually completingproduction of the n-type GaN composite substrates 10 each having a flatsurface, 2 inches of diameter, and 350 μm of the thickness as per across-sectional view shown in FIG. 1C and a plan view shown in FIG. 1D.Next, by further grinding the surfaces of the produced n-type GaNcomposite substrates 10, the surfaces of the produced substrates arefully leveled off to be ready for practical use. That is, in order toeffect epitaxial growth on the n-type GaN composite substrates 10, allthe surfaces are finished with a mirror polishing process.

The polished surface substantially satisfies the (0001) standard.However, in order to secure flat and satisfactory morphology of thenitride composite semiconductor layers superficially formed with theepitaxial growth, it is desired that a certain “off' angle be providedwithin a scope of 0.2 to 1° in the optional direction from the surface(0001), in particular, in order to minimize the superficial levelness,it is desired that the “off' angle be provided within a scope of 0.4 to0.8°.

As a result of the microscopic observation against the polished surfaceof the produced n-type GaN composite substrates 10, it was detected thatthe polished surfaces were not fully leveled off, but fine projectionsand recesses were generated. Concretely, there were locally hollowportions corresponding to those regions 24 that became the bottom of therecesses during the process for growing crystals shown in FIG. 1A.

Further, samples of the produced n-type GaN composite substrates 10 weresubjected to etching by immersing them in a blend solution containingsulfuric acid and phosphoric acid heated at 250° C. so that the etchedpits assembled with facets could be exposed externally. As a result, alarge number of etched pits emerged in the area corresponding to thebottom regions 24, and thus it was found that the “dislocation”phenomenon had extremely concentrated into the above area. It isconceived that, since the “dislocation” phenomenon had extremelyconcentrated into the area 24, this area was more vulnerable to erosionduring the polishing process, thereby resulted in the generation of thelocally hollow portions.

It was detected that the hollow generated local portion hadapproximately 10 to 40 μm of the width. Those portions other than theabove portion 24 were shared by such regions that contained low-densitydislocation corresponding to 10⁴ to 10⁵ cm⁻² of the etched pit density(EPD). The above portion 24 had contained the etched pit density by morethan three figures against other portions. These hollow portions 24individually correspond to the above-referred regions containinghigh-density crystalline defects in those portions each havingsubstantial density of crystalline defect (the density of the“dislocation”) by substantial figures greater than those which arepresent in the peripheral portions, and thus, these portions will bereferred to as the “regions concentrated with the dislocation”hereinafter in this specification.

It was detected that, unlike other regions on the n-type GaN compositesubstrate, there was the inversion of the polarity in the region 24concentrated with the dislocation. More particularly, although thoseregions other than the dislocation concentrated region 24 correspond tothe surface direction in which only the gallium (Ga) constituent isexternally exposed on the surface of the n-type GaN substrate 10, therewas a case in which nitrogen (N) constituent was externally exposed inthose regions 24 concentrated with the dislocation. Including theinversion of the polarity cited above, there are some aspects in thedislocation concentrated region 24. For example, there is such a case inwhich the dislocation concentrated region 24 consists of polycrystallinestructure. Further, there is such a case in which, despite of itsmono-crystalline structure, the region 24 slightly inclines againstperipheral regions containing crystalline defect with a low density.Further, there is such a case in which the axis “c” in the direction of[0001] is inverted against peripheral regions containing crystallinedefect with a low density. It should be noted that each of those regionsconcentrated with the dislocation has a definite interface so as todistinguish each of them from any of the peripheral regions.

Next, samples of the n-type GaN composite substrate 10 were irradiatedwith ultra-violet rays (it is possible to use 365 nm bright line via amercury (Hg) lamp), and then, luminescence from the sample surface wasobserved via a fluorescent microscope. As a result, it was observed thatthere were stripe-form regions having relatively clear interfaces at thecenter of those regions containing low-density dislocation sandwiched bythose regions 24 concentrated with dislocation with different contrasteffect from that of the peripheral regions. Those stripe-form regionsrespectively emit intense luminescence, which is more visible withslightly yellowish luminosity than in the case of the peripheralregions.

The observed region 25 emitting bright luminosity corresponds to theportion at which the surface {0001} followed growth as of the exposedstate on the way of growing crystals. The observed region 25 showed adifference from other portions in the observed effect. This isconceivably because there was a difference in the aspect of the induceddopant from that was generated in the peripheral regions. Based on theabove reason, this particular region 25 will be referred to as the“high-luminescence region” in the following description. Since theabove-referred portion at which the surface {0001} followed growth as ofthe exposed state on the way of growing crystals did not proceed withthe crystalline growth as of a constant width uniformly, althoughslightly being variable, it was evaluated that the width of thehigh-luminescence region 25 was in a range from 0 μm to a maximum of 30μm.

In order to implement a process for growing crystals required for theformation of the above-referred n-type GaN composite substrate 10, anyof the following methods other than the HVPE method may be applied,which include; a vapor phase growing method, the “Metalorganic chemicalvapor phase deposition (MOCVD)” method, the “Metalorganic chloride vaporphase epitaxy (MOVPE)” method, and a sublimation method.

In order to constitute the above-referred substrate 21 required for thegrowth of crystals for the formation of the n-type GaN compositesubstrate 10, not only the GaAs composite material, but it is alsopossible to use a mono-crystalline substrate capable of generatingsymmetry in the periphery of axes for three rounds or six rounds, inother words, it is possible to use hexagonal mono-crystals or cubicsymmetrical mono-crystals. By applying the (111) surface, the cubicsymmetrical mono-crystals can generate symmetry for three rounds. Any ofthose hexagonal mono-crystals including sapphire, SiC, SiO₂, NdGaO₃,ZnO, GaN, AlN, ZrB₂ may be used. Further, it is also possible to use asubstrate consisting of the cubic symmetrical (111) surface composed ofSi, spinnel, MgO, or GaP. These composite elements enable GaN compositematerial to be grown with the (0001) surface being the growing surface.

There are two methods for forming masks required for the formation ofthe n-type GaN composite substrate 10. One of these two method enablesmasks to be formed on the substrate 21 directly. In this case, it isrequired to arrange any effective means by way of depositing GaNcomposite buffering layers on the exposed surface of the substrateinside of window prior to the formation of the epitaxial layers. Theother method previously forms thin GaN composite layers on the substrateand then forms masks on the GaN composite layers. In many cases, thelatter method enables crystals to be grown very smoothly, and thus, thelatter method is preferred.

First Embodiment of the Present Invention

Referring now to the accompanying drawings, the first embodiment of thepresent invention for manufacturing a nitride composite semiconductorlaser element is described below, wherein the nitride compositesemiconductor laser element is produced by applying an n-type GaNcomposite substrate incorporating the above-referred regionsconcentrated with the “dislocated” crystalline defect and theabove-referred “high-luminescence regions”. FIG. 2 is a cross-sectionalview exemplifying the constitution of the nitride-compositesemiconductor laser element 1 according to the first embodiment of thepresent invention. Note that illustration of the “high-luminescenceregion” is not shown in FIG. 2.

[Process for Forming “Growth Inhibiting Films”]

First, a growth inhibiting film 13 shown in FIG. 2 is formed on thesurface of the n-type GaN composite semiconductor substrate 10. Thegrowth inhibiting film 13 is formed so as to cover a “dislocationconcentrated region” 11 (corresponding to the dislocation concentratedregion 24 shown in FIG. 1A to FIG. 1D) on the surface of the n-type GaNcomposite substrate 10. When constituting the nitride compositesemiconductor laser element of the present invention by laminating thenitride composite semiconductor layers on the n-type GaN compositesubstrate 10, the growth inhibiting film 13 prevents the dislocationfrom being succeeded within the film grown on the n-type GaN compositesubstrate 10. Due to this reason, the growth inhibiting film 13 shouldbe composed of a specific material that totally disables epitaxialgrowth of normal nitride composite semiconductor from the growthinhibiting film 13. To achieve this, the first embodiment of the presentinvention has introduced silicon dioxide SiO₂ for constituting thegrowth inhibiting film 13.

First, the n-type GaN composite substrate 10 is placed inside of anelectronic-beam-applied vapor deposition device. When the inner pressurehas reached a predetermined vacuum degree, by controlling the thicknessof silicon dioxide so as to become 0.2 μm, a SiO₂ film is formed on thesurface of the n-type GaN substrate 10. Next, availing of a simplelithographic etching method, the SiO₂ film complete with a vapor-phasedeposition process is treated with an etching process so as to solelyconceal the dislocation concentrated region 11 on the surface of then-type GaN composite substrate 10, thus forming the growth inhibitingfilm 13. Since the dislocation concentrated region 11 has a maximum of40 μm of the width, it is so arranged that the growth inhibiting film 13for covering the region 11 is provided with 50 μm of own width. Byimplementing the above arrangement, the GaN composite crystals are grownfrom a region 12 containing low-density dislocation.

The first embodiment has duly introduced silicon dioxide for composingthe growth inhibiting film 13. However, it is also allowable tointroduce similar silicon compound such as Si₃N₄ for example, ormetallic element such as tungsten (W), or titanium (Ti) as well. In thisembodiment, the thickness of the growth inhibiting film 13 is defined as0.2 μm. However, the growth inhibiting effect can be securedsufficiently by merely providing the film 13 with 0.05 μm up to 1.0 μmof the thickness. Further, in this embodiment, width of the growthinhibiting film 13 is defined as 50 μm. However, as far as the growthinhibiting film 13 has a width enough for covering the dislocationconcentrated region 11 and for allowing further progress of epitaxialgrowth of a normal nitride composite semiconductor in the low-densitydislocation region 12, the growth inhibiting film 13 may be providedwith a wider width.

[Epitaxial Growth of the Nitride Composite Semiconductor Layers]

Using an MOCVD processing device, n-type GaN composite layer 101 having3 μm of film thickness is formed at 800° C. of substrate temperature onan n-type GaN composite substrate 10 by applying NH₃ as the V-familysource, TMGa (trimethyl-gallium) or TEGa (triethyl-gallium) as the IIIfamily source, and SiH₄ as dopant in presence of carrier gas comprisinghydrogen or nitrogen. Next, a crack-preventing layer 102 composed ofn-type of In_(0.07)Ga_(0.93)N is formed by 40 nm by adding TMIn(trimethyl-indium) as the III family source to the above-referredcomposite material at 800° C. of the substrate temperature.

Next, by heating the substrate up to 1050° C., using TMAl(trimethyl-aluminum) or TEAl (triethyl aluminum) as the III familysource, a clad layer 103 compound of n-type Al_(0.1)Ga_(0.9) N having1.2 μm of film thickness is formed. The dopant material was adjusted sothat silicon as the n-type impurities could correspond to 5×10¹⁷ cm⁻³ to1×10¹⁹ cm⁻³ in its composition. Next, n-type GaN light guide layer 104having 0.1 μm of film thickness is formed, in which density of thesilicon impurities is defined to be 1×10¹⁶ to 1×10¹⁸ cm⁻³.

Then, the substrate temperature is lowered to 750° C., and an activelayer consisting of the multiple quantum well structure 105 is formed.This layer 105 includes a barrier layer, a well layer, a barrier layer,a well layer, a barrier layer, a well layer, and a barrier layer, andthus has tertiary periodicity. The well layers are eachIn_(0.1)Ga_(0.9)N with 4 nm of film thickness, and the barrier layersare each In_(0.01)Ga_(0.99)N with 8 nm of film thickness. In the courseof forming only the barrier layer or both the barrier layer and the welllayer, SiH₄ is introduced, where the density of the silicon impuritiesis 1×10¹⁶ to 1×10¹⁸ cm⁻³. At the time of switching the formation of thebarrier layer and the well layer, if the growth process were suspendedfor a minimum of one second up to a maximum of 180 seconds, levelness ofindividual layers is promoted, and conversely, width of half-valueluminous layer decreases favorably.

Arsenic material addable to the above active layer 105 includes thefollowing: AsH₃ (arsine), or TBAs (tertiary butyl arsine), or TMAs(trimethyl arsine). Phosphoric material addable to the active layer 105includes the following: PH₃ (phosphines), or TBP (tertiary butylphosphine), or TMP (trimethyl phosphine). Antimony material addable tothe active layer 105 includes TMSb (trimethyl antimony), or TESb(triethylene antimony). In the course of forming the active layer 105,as practically available nitrogen material, not only NH₃, but it is alsoallowable to use hydrazines such as N₂H₄ (hydrazine) and C₂N₂H₈(dimethyl hydrazine), or azides such as ethyl azide.

When the active layer 105 incorporates plural layers of quantum wellscomprising In_(x)Ga_(1-x)N, and also when adding arsenic or phosphor tothe active layers 105 to make it composed of active quantum layers, itis known that, if dislocation running through the quantum wells ispresent, indium segregates to the portion in which dislocation ispresent. Accordingly, when applying quantum wells mainly comprising theabove-mentioned In_(x)Ga_(1-x)N to the active layers 105, in order tosecure better laser effect, it is essential that occurrence of thedislocation (crystalline defect) be minimized as far as possible.

Next, the substrate is again heated up to 1050° C., and then areserially formed a p-type Al_(0.3)Ga_(0.7)N carrier block layer 106 with20 nm of film thickness, a p-type GaN light guide layer 107 with 0.1 μmof thickness, a p-type Al_(0.1)Ga_(0.9)N clad layer 108 with 0.5 μm ofthickness, and a p-type GaN contact layer 109 with 0.1 μm of thickness.In this process, as the p-type impurities, EtCP₂Mg (bis-ethylcyclopentadienyl magnesium) is used, and the concentration of magnesiumis adjusted to be 1×10¹⁸cm⁻³ to 2×10²⁰ cm⁻³. As the magnesium material,it is allowable to use other cyclopentadienyl compounds such ascyclopentadienyl magnesium and bis-methyl cyclopentadienyl magnesium.

In terms of the concentration of p-type impurities in the p-type GaNcomposite contact layer 109, it is preferred that the concentration ofp-type impurities be strengthened in the direction of the positiveelectrode 15. This arrangement will cause contact resistance toattenuate itself when forming the positive electrode 15. Further, inorder to eliminate residual hydrogen within the p-type layer obstructingmagnesium as the p-type impurities from being activated, a negligibleamount of oxygen may be infused in the course of growing the p-typelayer.

After completing formation of the p-type GaN composite contact layer109, atmosphere inside of the reactor of the MOCVD device is thoroughlyreplaced with nitrogen carrier gas and NH₃, and then the temperature ofthe substrate is lowered at a rate of 60° C. per minute. When thetemperature of the substrate reaches 800° C., supply of NH₃ isdiscontinued. Then, the substrate temperature is held on at 800° C. for5 minutes before eventually lowering the substrate temperature to roomtemperature. It is preferred that the intermediate substrate temperaturebe held on in a range from 650° C. up to 900° C. for a minimum of 3minutes and a maximum of 10 minutes. It is also preferred that thesubstrate temperature be lowered to room temperature at a rate of 30° C.per minute or higher.

Then, the nitride composite semiconductor layers complete with the aboveprocesses is subject to evaluation via Raman measuring method. Eventhough the wafers extracted from the above MOCVD processing device hasnot been treated with an annealing process for conversion into thepositive type, since magnesium component has already been activated,immediately after the growth, the produced layers demonstrate specificcharacteristics proper to the positive type. Further, contact resistancecaused by formation of the positive electrode 15 is lowered. Inparticular, by way of combining with a conventional annealing processfor conversion into the positive type, efficiency in the activation ofmagnesium is favorably promoted.

In the n-type crack-prevention layer 102, which is In_(0.07)Ga_(0.93)Nin the above example, indium composite ratio may not be 0.07. Further,it is also permissible to omit the n-type InGaN crack-prevention layer102 itself. However, if incompatibility of the lattice constitutionbetween the n-type AlGaN clad layer 103 and the n-type GaN substrate 10grows, for the sake of preventing crack from occurrence, it is preferredthat the n-type InGaN crack prevention layer 102 be inserted.Alternatively, for the sake of preventing crack from occurrence, Ge(germanium) may be used in place of silicon as the n-type impurities.

Composite constitution of the above-referred active layer 105 iscommenced with a barrier layer and completed with a barrier layer.Alternatively, composition of the active layer 105 may also be commencedwith a well layer and completed with a well layer. The number of thewell layer is not solely limited to three. When the number of the welllayers is ten or less, current density at a threshold value remains low,thereby making it possible to generate oscillation continuously at roomtemperature. In this case, when a minimum 2 of well layers up to amaximum of 6 layers are provided, current density at a threshold valuestably remains low, and thus, this constitution is particularly suitablefor practical use. Further, it is also allowable to have the aboveactive layer 105 contain aluminum.

Though the above description has solely referred to a constitution ofthe active layer 105 consisting of the well layers and the barrierlayers respectively added with a specific amount of silicon asimpurities, addition of the impurities to those layers may be omitted.However, luminous intensity is promoted when impurities are added to theactive layer 105. Aside from silicon, any of those elements includingoxygen, carbon, germanium, zinc, and magnesium, may be used forconstituting impurities. It is preferred that a sum of addableimpurities be arranged at approximately 1×10¹⁷ to 8×10¹⁸cm⁻³. Further,it is also allowable to add impurities to either of the well layers andthe barrier layers instead of both layers.

Constitution of the p-type Al_(0.3)Ga_(0.7)N carrier block layer 106 maydiffer from this composition. For example, when indium is added, thecarrier block layer can be grown under lower temperature into thepositive type, thus enabling the above layer to be grown on a substratemaintained at a low temperature. This method minimizes potential risk ofdamaging the active layer 105 on the way of growing crystals. Althoughthe p-type AlGaN carrier block layer 106 may not be provided, presenceof this layer 106 lowers current density at a threshold value. This isbecause the p-type AlGaN carrier block layer 106 functions itself so asto confine carrier component with the active layers 105.

Further, by increasing composite ratio of aluminum in the p-type AlGaNcarrier block layer 106, degree of confining carrier in the active layer105 is favorably promoted. In this case, by further decreasing compositeratio of aluminum down to a critical degree merely capable of sustainingthe effect of confining carrier, this in turn causes the mobility of thecarrier inside of the p-type AlGaN carrier block layer 106 to beenhanced to result in the lowered electrical resistance. Accordingly,this method is preferred for practical use.

As the n-type AlGaN clad layer 103 and the p-type AlGaN clad layer 108,crystals comprising Al_(0.1)Ga_(0.9)N are used. In place of this, it isalso allowable to use ternary crystals comprising AlGaN in which the Alcomposite ratio is other than 0.1. When raising the composite ratio ofaluminum, difference in the energy gap and the refraction index expandsfrom those of the active layer 105 to make it possible to effectivelyconfine carrier and light within the active layer 105, thereby loweringcurrent density at a threshold value in the laser oscillation. Further,by decreasing the composite ratio of aluminum further down to a criticaldegree capable of merely sustaining the effect of confining carrier andlight, it will cause the mobility of the carrier to be enhanced in then-type AlGaN clad layer 103 and the p-type AlGaN clad layer 108, therebylowering operating voltage of the corresponding element.

In this case, by arranging film thickness of the n-type AlGaN clad layer103 in a range from 0.7 μm to 1.5 μm, effect of making the verticaltransverse mode single-peaked and efficiency of confining light can beenhanced, thereby enabling to improve optical characteristics of laserand lower current density at a threshold value of laser. In the aboveexample, the above-referred n-type clad layer 103 and the p-type cladlayer 108 are respectively constituted into ternary composite crystalscomprising AlGaN. Alternatively, it is also possible to prepare then-type and p-type clad layers 103 and 108 with quarternary compositecrystals such as AlInGaN, AlGaNP, or AlGaNAs, for example. Further, inorder to lower electric resistance, it is also allowable to constitutethe p-type clad layer 108 with a super-lattice structure consisting of ap-type AlGaN layer and a p-type GaN layer or with another super-latticestructure consisting of a p-type AlGaN layer and a p-type InGaN layer.

The above description has solely referred to the method of growingcrystals by applying the MOCVD processing device in the course oflaminating nitride-composite semiconductor layers on the substrate 10comprising n-type GaN composite element. Alternatively, it is alsopossible to introduce a “molecular beam epitaxy” (MBE) method or theabove-cited HVPE (hydride vapor phase epitaxy) method.

Through the above serial processes, epitaxial wafers are obtained, whicheach have the layers of the nitride composite semiconductor on then-type GaN composite substrate 10. These epitaxial wafers are thenextracted from the MOCVD processing device for the ensuing processes forconverting them into individual chips of the nitride compositesemiconductor laser elements. Note that, as shown in FIG. 2, the p-typeAl_(0.1)Ga_(0.9)N clad layer 108 is formed with a projected ridge-stripeconfiguration, which is complete with a processing step to be describedlater on.

Except for a local portion right above the growth inhibiting film 13,surface of each epitaxial wafer produced via steps for processing thenitride composite semiconductor elements is fully leveled off. Notethat, at a local portion right above the growth inhibiting film 13, thenitride-composite semiconductor layers cannot grow themselvesepitaxially, and forms a recess. After completing formation of thenitride-composite semiconductor layers, it was confirmed that no crackwas generated on those portions complete with the epitaxial growth(these portions will be referred to as the “epitaxially grown portionshereinafter) except for a local portion right above the growthinhibiting film 13.

It is conceived likely that, since the recessed portion right above thegrowth inhibiting film 13 relaxes stress, residual stress in theepitaxially grown portion is lowered in the direction perpendicular tothe ridge-stripe portion (in the leftward and rightward direction shownin FIG. 2). Further, although the effect of dislocation spreads up tothe surface of the GaN composite substrate, due to presence of thegrowth inhibiting film 13, the effect of dislocation is prevented fromfurther diffusing into the epitaxially grown portion on the GaNcomposite substrate 10. Accordingly, density of the dislocation islowered inside of the nitride-composite semiconductor layers furtherthan the case of using a normal GaN substrate 10 without exceedingdensity of dislocation in the region containing a low degree of defectin the substrate.

It was described earlier that polarity on the surface of the GaNsubstrate 10 may be inverted so that the nitrogen appears on the surfacein the region 11 concentrated with dislocation. However, as a result ofinitially forming the growth inhibiting film 13 on the region 11concentrated with dislocation followed by a process for growing nitridesemiconductor layers, nitrogen surface coexists on the surface of theGaN substrate 10, thus growth on a normal gallium surface is notretarded. Accordingly, as in the case in which polarity of the region 11concentrated with dislocation remains without being inverted, asspecific characteristics proper to the nitride semiconductor laserelement, effect resulted from the decrease of crystalline defect andrelaxed stress can be confirmed.

(Process for Conversion into Elements)

A ridge-stripe portion is formed on the surface of flat portion of thenitride composite semiconductor layers in order to confine light in thehorizontal direction against the n-type GaN substrate 10. However, whenusing the n-type GaN substrate containing high luminescence region 25shown in FIG. 1A to 1D, it is desired that the ridge-stripe portion notbe formed right above the high luminescence region 25. This because ofthe following undesirable reasons. Concretely, it is not desired tocause drive current to flow through the high luminescence region 25 dueto the rise of drive current flowing into the nitride semiconductorlaser elements and also due to the rise of voltage borne by the elementscaused by the rise of resistivity and a less amount or lower degree ofactivation of dopant in the high luminescence region 25 than that ofother regions.

The ridge-stripe portion is formed by executing an etching process fromthe surface of the epitaxial wafer to an intermediate portion of thep-type AlGaN clad layer 108 except for the striped portion having 1 to 3μm, preferably 1.3 to 2 μm of width. The distance from the p-type GaNguide layer 107 at the etched bottom surface was arranged to be from 0to 0.1 μm. In the following step, an insulating film 110 made of AlGaNcomposite material is formed-except for the ridge-stripe portion. Sincethe p-type GaN contact layer 109 without being subject to the etchingprocess remains exposed, a positive electrode 15 is formed on thesurfaces of the p-type GaN contact layer 109 and the insulating film 110via a vapor-phase deposition process in the sequence of palladium (Pd),molybdenum (Mo), and gold (Au).

Aside from AlGaN, it is also allowable to use any of the followingmaterials for composing the insulating film 110 including oxide ornitride of silicon, titanium, zirconium, tantalum, and aluminum.Further, any of those composite elements including palladium(Pd)/platinum (Pt)/gold (AU), or palladium (d)/gold (Au), or nickel(Ni)/gold (Au), may also be used for composing the positive electrode15.

Further, by grinding the back surface side (on the substrate side) ofthe epitaxial wafer provided with the positive electrode 15, thicknessof the epitaxial wafer is adjusted to be 80 μm to 200 μm, therebyfacilitating division of the epitaxial wafer afterwards.

Next, a negative electrode 16 is formed on the back of the n-type GaNsubstrate 10 in the sequence of hafnium (Hf) and aluminum (Al). It isallowable to use any of those composite elements cited below forcomposing the negative electrode 16 including the following: hafnium(Hf),/aluminum (AD/molybdenum (Mo)/gold (Au), hafnium (HP/aluminum(AD/platinum (Pt)/gold (Au), hafnium (HP/aluminum (Al)/tungsten (W)/gold(Au), hafnium (Hf)/gold (Au), hafnium (Hf)/molybdenum (Mo)/gold (Au). Itis also allowable to use any of the above-cited composite materials forcomposing the negative electrode 16 by way of substituting hafnium withtitanium (Ti) or zirconium (Zr).

Finally, by causing the epitaxial wafer provided with the negativeelectrode 16 to be split in the direction perpendicular to theridge-stripe direction via cleavage, a Fabry-Perot resonator with 600 μmof resonating length is prepared. It is desired that length of theresonator shall be a minimum of 300 μm to a maximum of 1000 μm. Aftercompleting the above serial processes, the epitaxial wafer becomes abar-like configuration in which individual nitride-compositesemiconductor laser elements are aligned in the horizontal direction. Anedge surface of the resonator of the nitride semiconductor laser elementhaving its stripe form oriented in the <1-100> direction constitutes the{1-100} surface. Cleavage is not implemented after forming ruled lineall over the epitaxial wafer via a scriber, but the cleavage iscommenced at an origin after putting ruled lines at both ends of theepitaxial wafer for example via a scriber.

Not only the above-described feedback method based on the edge surfaceof the resonator by applying the above bar-like Fabry-Perot resonator,but it is also allowable to apply the “distributed feedback (DFB)” unitprovided with a conventionally known diffraction lattice inside of theresonator or the “distributed Bragg reflector (DBR)” unit provided witha diffraction lattice outside of the resonator. After completing asplitting process via cleavage followed by formation of the edge surfaceof the bar-like Fabry-Perot resonator, dielectric films composed of SiO₂containing 70% of reflection index and TiO₂ are alternately deposited onthe edge surface of the resonator via vapor-phase deposition, thuscompleting formation of multiple dielectric reflection films. It is alsoallowable to use composite elements such as SiO₂/Al₂O₃ for composing themultiple dielectric reflection films.

Constitution of the above bar-like Fabry-Perot resonator is variable bythe difference in terms of the film thickness or growth condition of thenitride-composite semiconductor layers laminated on the n-type GaNcomposite substrate 10, where the growth condition includes substratetemperature during growth and pressure inside of a reaction furnace. Forexample, if each of the nitride composite semiconductor layers is merelyprovided with a thin film thickness, then, as shown in FIG. 3, nitridecomposite semiconductor layers 402 cannot be bonded right above thegrowth inhibiting films 13, thereby causing region 403 right above thegrowth inhibiting films 13 to be separated from other portions. Aportion formed in a region between the growth inhibiting films 13constitutes a unit of the nitride-composite semiconductor laser element401.

When the film thickness of the nitride composite semiconductor layershas been increased from that is shown in FIG. 3, the actual aspect lookslike the one shown in FIG. 4 or 5. Concretely, in FIG. 4, althoughnitride composite semiconductor layers 502 are bonded together rightabove the growth inhibiting film 13, crack arriving at the surface ofthe semiconductor layers 502 is present in a region 503 right above thegrowth inhibiting film 13. Further, as shown in FIG.4, hollow portion504 may be generated on the surface of the growth inhibiting film 13within the region 503. When this condition is present, a portion formedin a region between individual growth inhibiting films 13 constitutes anitride composite semiconductor laser element 501.

FIG. 5 represents a state in which nitride-composite semiconductorlayers 603 are fully bonded with each other right above the growthinhibiting film 13. Accordingly, unlike the one shown in FIG. 4, it isconfirmed that no crack is present in a region 603 right above thegrowth inhibiting film 13 otherwise arriving at the surface of thenitride composite semiconductor layers 602. Further, as shown in FIG. 5,a hollow portion 604 may be generated on the surface of the growthinhibiting film 13 within the region 603. A portion formed in a regionbetween individual growth inhibiting films 13 constitutes a unit ofnitrogen composite semiconductor laser element 601.

Then, by causing the above bar-like Fabry-Perot resonator to be divided,the nitride-composite semiconductor laser elements according to theconstitution shown in FIG. 2 are eventually secured. During the dividingprocess, a laser beam guide region 14 is disposed at the center of theabove semiconductor laser elements right beneath the ridge-stripeportion before dividing the above resonator so as to provide each of thesemiconductor laser elements with 400 μm of lateral width. Originally, acertain number of specific regions 24 each being concentrated withdislocation is disposed in the n-type GaN substrate 10 at 400 μm ofintervals (pitch P) as shown in FIG. 1A to 1D. It is suggested that,when securing the nitride-composite semiconductor laser elements bydividing the above-referred bar-like Fabry-Perot resonator, it willgenerate much convenience by dividing lateral width of the semiconductorlaser element into the width of 1 for the integral multiple of the pitchP of a dislocation concentrated region, or an integral multiple.

By serially executing the above processes, the nitride compositesemiconductor laser element chips as shown in FIG. 2 are thus completed.Accordingly, by applying the n-type GaN substrate 10 with intentionalcontrol over the dislocation substantially being crystalline defectcontained therein, and further, by fully covering the dislocationconcentrated region 11 with the growth inhibiting film 13, a laser beamguide region 14 is formed in the region incurring negligible effect ofdislocation and a low degree of stress in order that the laser beamguide region 14 will become a stricture portion of the current flowingthrough the above semiconductor laser elements. Accordingly, based onthe above arrangement, it was eventually possible to sustain more than5000 consecutive hours of the laser oscillation service life under thecondition of 60 mW of laser output power and 70° C. of environmentaltemperature.

It should be noted that the film thickness of the above growthinhibiting film 13 was rated as 0.2 μm in this embodiment. However,applicable film thickness is not solely limited to this value. A certainnumber of nitride composite semiconductor laser elements have beenformed that have different film thickness of the growth inhibiting film13 ranging from 0.01 μm up to 2.0 μm. When the growth inhibiting film 13is less than 0.05 μm of thickness, the growth inhibiting film 13 incursdamage due to the rise of temperature before implementing lamination ofthe above semiconductor layers, thus failing to secure growth inhibitingeffect.

Further, when the growth inhibiting film 13 is thicker than 0.1 μm, dueto the difference in the thermal expansion coefficient between then-type GaN composite substrate 10 and the growth inhibiting film 13, therise of temperature before laminating the nitride-compositesemiconductor layers also causes the growth inhibiting film 13 to incurdamage. Even if the growth inhibiting film 13 itself does not incurdamage, nitride semiconductor layers directly growing on the surface ofthe GaN composite substrate 10 other than the growth inhibiting film 13may be subject to abnormal growth in the vicinity of the heightdifference between the growth inhibiting film 13 and the GaN compositesubstrate 10, due to obstruction generated by diffused material causedby presence of the height difference. Generation of this kind ofabnormal growth is called “edge effect” Due to generation of the “edgeeffect”, obstruction may be generated during the production processes,or the laser oscillation wave-length may vary or oscillation thresholdvalue may rise.

Based on the above reasons, after forming the growth inhibiting film 13by a minimum of 0.05 μm and a maximum of 1 μm, practical effect of thegrowth inhibiting film 13 can be secured, thus enabling the nitridesemiconductor laser elements incorporating the above growth inhibitingfilm 13 to secure durable service life under high-output ageing.

Second Embodiment of the Present Invention

Referring now to the accompanying drawings, the second embodiment of thenitride semiconductor laser elements manufactured by applying the n-typeGaN composite substrate incorporating those regions concentrated with“dislocation” and those high luminescence regions based on the aboveformation is described below. Processes for epitaxially growing thenitride semiconductor layers and manufacturing laser elements areidentical to those which are performed for the first embodiment of thepresent invention, and thus, for detailed description of theseconstituents, refer to the preceding description on the firstembodiment, thereby omitting description thereof.

In the second embodiment, unlike the preceding first embodiment, agrowth inhibiting film is formed in order that those regions 24concentrated with “dislocation” (shown in FIG. 1A to 1D) can fullycovered by the growth inhibiting film, and that the growth inhibitingfilm can be formed on the n-type GaN composite substrate 10 in thedrain-board form. Concretely, as was performed in the first embodiment,initially, an n-type GaN composite substrate 10 is placed inside of anelectron-beam-applied vapor-phase deposition device, and then, afterenabling the inner pressure to arrive at a specific vacuum degree, byprocessing silicon dioxide into 0.2 μm of film thickness a SiO₂ film isformed on the surface of the n-type GaN composite substrate 10. Next,using a simple lithographic method, as shown in FIG. 6A, thevapor-deposited SiO₂ is formed so as to constitute a growth inhibitingfilm 301 split into three parts across 5 μm of intervals each having 10μm of width by way of covering those regions 11 concentrated withdislocation on the n-type GaN composite substrate 10.

As shown in FIG. 6A, by executing the above serial processes, aplurality of growth inhibiting films 301 each having 0.2 μm of thicknessare formed on the regions 11 concentrated with dislocation inside of then-type GaN composite substrate 10. Next, by applying the MOCVDdeposition device, an n-type GaN composite film is formed in presence ofnormal atmospheric pressure and at 1000° C. of substrate temperature. Asa result, as shown in FIG. 6B, GaN composite crystals 302 growthemselves in the direction of [0001] through windows each having 5 μmof opening between a plurality of growth inhibiting films 301. When thiscondition is present, sectional shape of the GaN composite crystals 302remains in the projected triangular form. Next, by lowering pressure ofa reaction furnace of the MOCVD deposition device to 70 Torr, a processfor growing the GaN crystals 302 is implemented again at 1080° C. of thesubstrate temperature.

When the above process is underway, the n-type GaN composite crystals304 shown in FIG. 6B correspond to the portions grown from bare portionwithout being covered by the growth inhibiting films 301. The innerportion of the corresponding n-type GaN composite film 304 merelycontains a specific density of dislocation substantially correspondingto that is present in those regions other than those regions 11concentrated with dislocation inside of the n-type GaN compositesubstrate 10. The GaN composite crystals 302 contains the dislocation303 which is present in succession to the parallel direction against thegrowth axis in the direction of [0001]. However, when the growth beginsin the lateral direction perpendicular to the growth axis in thedirection of [0001], the dislocation 303 bends itself in the directionperpendicular to the direction [0001]. At the same time, (11-22) and(-1-122) facets grow themselves as the foremost tip portions.

Thus, lateral directional growth is promoted from the portion at whichthe GaN composite crystals grew themselves in the form of projection asshown in FIG. 6C, and then, as shown in FIG. 6C, the GaN compositecrystals are united together at the center portion right above thegrowth inhibiting films 301 and further grow themselves upward. Further,by causing an n-type GaN composite film 304 containing low-dislocationregions to be grown in the lateral direction, an n-type GaN compositefilm 305 is formed, which covers a GaN epitaxial film generated via thegrowth of the projected GaN composite crystals 302. The GaN epitaxiallayer covered with the n-type GaN composite film 305 incorporates adislocation 306 bent via the lateral-directional growth. Further, aslight amount of dislocation concentrates into a junction portion 307linked with the GaN composite crystals 302 and the n-type GaN compositefilm 304 as a result of the lateral-directional growth.

The above art was described in the paper prepared for the 22nd and 171stjoint meeting of two committees of Japan Society for the Promotion ofScience, namely No. 162 Short-Wavelength Optical Device Committee andNo. 125 Photoelectric Conversion Committee (Dec. 15 to 16, 2000), onpages 25 to 32.

After growing the GaN composite crystals on the surface of the n-typeGaN composite substrate 10, the surface of the GaN composite film 305with a sum of approximately 8 μm of film thickness was fully leveledoff. After growing the n-type GaN composite film 305, by applying themethod identical to that was applied to the first embodiment, layerstructure for constituting the nitride semiconductor laser element wasserially grown via crystalline formation. Consequently, the nitridesemiconductor layers 308 were eventually formed on the n-type GaNcomposite film 305.

After executing a final etching process, a ridge stripe portion 309 wasformed so as to complete a laser element. The position for forming theridge stripe portion 309 in the nitride semiconductor laser element maybe provided above those local regions 12 accommodating low-densitydislocation other than the regions right above those regions 11concentrated with dislocation, or it may be provided right above thegrowth inhibiting film 303. By virtue of the above arrangement, afterexecuting the second embodiment, as a result of the lowered crystallinedefect and the relaxed stress, it was possible to secure such nitridesemiconductor laser elements incorporating as satisfactory physicalcharacteristics as that was secured by execution of the firstembodiment.

In the second embodiment, width of the growth inhibiting film 301 isdefined to be 10 μm and the intervals between individual growthinhibiting films to be 5 μm. It should be noted that, insofar as thegiven width and intervals enable selective growth and lateraldirectional growth of the growth inhibiting film 301, superiority interms of characteristics of the nitride semiconductor laser element canbe admitted. It was confirmed that, when the width given to the growthinhibiting film 301 and the intervals between adjoining growthinhibiting film were respectively a minimum of 1 μm and a maximum of 10μm, it was possible to secure such a nitride semiconductor laser elementwith the most desirable characteristics. The thickness of the GaNcomposite film 305 formed for covering the growth inhibiting film 301 isdefined to be 8 μm. However, by arranging the thickness of the GaNcomposite film 305 in a range from 1 μm to a maximum of 20 μm, it ispossible to lower the influence of the growth inhibiting film 301against the laminated nitride semiconductor layers.

Further, the second embodiment has introduced silicon dioxide forconstituting the growth inhibiting film 301. However, as was allowed inthe first embodiment, it is also possible to use other silicon compoundsuch as Si₃N₄ or metallic element such as tungsten (W) or titanium (Ti)or the like. Further, although thickness of the growth inhibiting film301 for covering the substrate was defined to be 0.2 μm, it is alsoallowable to provide 0.05 μm up to a maximum of 1 μm of its thickness.Further, the drain-board shaped growth inhibiting film 301 is soarranged that the width corresponding to the sum of the width itself andintervals between adjoining growth inhibiting films 301 can fully coverall the underlying regions 11 concentrated with the dislocation.

The present description on the first and second embodiments of theinvention has thus far solely referred to the ridge-stripe type nitridesemiconductor laser elements. However, scope of the first and secondembodiments is not solely limited to the above ridge-stripe type. Thedescription has also referred to an example of forming electrodes on theback surface of the n-type GaN composite substrate 10 and on the frontsurface of a nitride semiconductor film formed on the surface of then-type GaN composite substrate. Alternatively, it is also allowable toform the p-type and n-type electrodes on the surface of the n-type GaNcomposite substrate 10.

[The Case in which an Elementary Group of As, P, and Sb, is Included inthe Active Layer]

The semiconductor laser element having the constitution shown in FIG. 2manufactured via the processes according to the first or secondembodiment of the present invention consists of the laminate of an InGaNcomposite well layer and an InGaN composite barrier layer forconstituting the active layer 105. The active layer 105 may include atleast any of the elements among an elementary group including arsenic(As), phosphor (P), and antimony (Sb).

In this case, it is so arranged that at least any of the elements amongan elementary group including As, P, and Sb, be included in the welllayer among the active layer 105 constituting the above nitridecomposite semiconductor laser element. In this case, assume that thecomposite ratio of a sum of the elementary group comprising As, P, andSb included in the well layer is defined as “X′, whereas the compositeratio of nitrogen element is defined as “Y”, it is so arranged that X issmaller than Y, and yet, X/(X+Y) is less than or equal to 0.3. It isfurther preferred that X/(X+Y) be less than or equal to 0.2.

The lower limit value of the sum of the elementary group is defined tobe a minimum of 1×10¹⁸cm⁻³. If the composite ratio X of the sum of theabove elementary group becomes higher than 0.2, concentrationseparation, i.e., a phenomenon in which the composition of theabove-mentioned elements varies from one region to another within thequantum well layer, starts to occur. Further, if the composite ratio ofthe sum of the above elementary group becomes higher than 0.3, it willcause the above concentration separation to shift to the crystallineseparation causing hexagonal crystals and cubic crystals to beintermingled with each other, thereby lowering the crystallizationeffect of the well layer. On the other hand, if the sum of the aboveelements added to the well layer is less than 1×10¹⁸cm⁻³, it will becomedifficult to secure proper effect of including the above elements in thewell layer.

As described above, by causing As, P, and Sb, to be included in theactive layer 105, effective mass of electrons and holes of the welllayer decreases, and conversely, mobility of electrons and hole of thewell layer increases. Accordingly, in the case of the nitride compositesemiconductor laser element, decrease of the effective mass of electronsand holes of the well layer enables to secure inverted distribution ofcarrier for the laser oscillation by feeding a small amount of current.Further, increase of the mobility of electrons and holes of the welllayer generates such an effect of enabling electrons and holes todiffuse themselves over again and then infused into the well layer at ahigh velocity even after electrons and holes were subject to extinctionin the well layer once due to generation of illuminant reunion.

It is known that the above advantageous effects appear mostconspicuously when crystalline defect is not present in the quantumwell. Concretely, in contrast with such an InGaN composite nitridesemiconductor laser element devoid of any elementary constituent in theactive layer 105, by causing any of the elements As, P, or Sb, to beincluded in the active layer 105, it is possible to produce qualitynitride composite semiconductor laser element featuring low thresholdcurrent density and excellent self-excited oscillation characteristics,i.e., excellent in the noise suppression characteristics.

In the first and second embodiment, prior to the epitaxial growth of thenitride composite semiconductor layers, a growth inhibiting film isformed by way of covering the dislocation concentrated regions on thesurface of the GaN composite substrate. However, in the followingembodiments described hereinafter, the epitaxial growth of the nitridecomposite semiconductor layers is solely implemented without forming thegrowth inhibiting film on the surface of the GaN composite substrate.However, in order to prevent the influence of the dislocationconcentrated regions inside of the GaN composite substrate fromadversely affecting the laser beam guide region, a specific relationshipis arranged at a relative position between them.

Third Embodiment of the Present Invention

FIG. 7 is a schematic cross-sectional view of a semiconductor laserelement 3 according to the third embodiment of the present invention. InFIG. 7, the semiconductor laser element 3 of the third embodiment isseen from the light emitting direction. The reference numeral 10represents an n-type GaN composite substrate, in which a number ofdislocation concentrated regions 11 are present. Those portions otherthan the dislocation concentrated region 11 consist of low-dislocationaccommodating regions. Intervals (pitch P) between individual regions 11are respectively at 400 μm. Within the low dislocation regions, highluminescence regions 25 are present in parallel with the regions 11. Anumber of nitride semiconductor (epitaxially grown) layers 1000 areformed on the substrate 10. A laser beam guide region 14 is disposedinside of the nitride semiconductor layers. An electrode 15 is formed onthe upper surface of the nitride semiconductor layers 1000, whereas theother electrode 16 is disposed beneath the substrate 10. Although notshown in this cross-section view, the dislocation concentrated regions11, the high-luminescence regions 25 and the laser beam guide region 14,are respectively extended into the depth direction of FIG. 7, which aredisposed substantially in parallel with each other.

According to a plan view of the semiconductor laser element 3 of thethird embodiment, the distance (d) between the laser beam guide region14 and the dislocation concentrated region 11 is defined at 80 μm,whereas the distance (t) between the laser beam guide region 14 and thehigh-luminescence regions 25 is defined at 120 μm. In the presentspecification, the distances “d” and “t” shall respectively bedetermined by referring to the distance between the center positions ofthe stripe-form laser beam guide region 14, regions 11 concentrated withdislocation, and the high-luminescence regions 25. Further, in thefollowing embodiments, the center portion corresponding to thehigh-luminescence regions 25 within the low-dislocation region will bereferred to as the “center portion of the low dislocation region”. It ispossible to consider those portions described as the high luminescenceregions 25 by replacing with the “center portion of the low dislocationregion”.

[Epitaxial Growth of Nitride Semiconductor Layers]

Referring now to FIG. 8, a method of manufacturing the semiconductorlaser element 3 by forming nitride semiconductor layers 1000 on ann-type GaN composite substrate 10 is described below. Note that FIG. 8is a schematic diagram for representing the detail of the layerconstitution of the nitride semiconductor layers 1000 by referring tothe semiconductor laser element shown in FIG. 7. Note further that,since the constitution and the method of manufacturing the GaN compositesubstrate 10 have previously been described, further description on theconstitution of the substrate 10 is deleted.

Initially, using an MOCVD deposition device, dopant material composed ofSiH₄ was added to NH₃ which is the V family source and TMGa (trimethylgallium) or TEGa (triethyl gallium) which is the III family source.Next, an n-type GaN composite layer 1102 having 3 μm of film thicknesswas formed on an n-type GaN substrate 10 at 1050° C. of the substratetemperature. Next, TMIn (trimethyl indium) as the III family source wasadded to the above compound material to form an n-typeIn_(0.07)Ga_(0.93)N crack-preventing layer 1103 by 40 nm via growth.Next, the substrate 10 was heated up to 1050° C., and then, using TMAl(timethyl aluminum) or TEAl (triethyl aluminum) as the III familysource, an n-type Al_(0.1)Ga_(0.9)N clad layer 1104 having 1.2 μm ofthickness was formed via growth. In this process, impurities consistingof silicon were added by an amount corresponding to 5×10¹⁷ cm⁻³ to1×10¹⁹ cm⁻³. Next, an n-type GaN composite beam guide layer 1105 wasformed by 0.1 μm of thickness via growth by adding silicon impurities byan amount corresponding to 1×10¹⁶ to 1×10¹⁸ cm⁻³.

Next, the substrate temperature was lowered to 750° C., and then activelayer 1106 consisting of multiple quantum well was formed. This layerincludes a barrier layer, a well layer, a barrier layer, a well layer, abarrier layer, well layer, and a barrier layer, and thus has tertiaryperiodicity. The well layers are In_(0.1)Ga_(0.9)N with 4 nm ofthickness, and the barrier layers are In_(0.01)Ga_(0.99)N with 8 nm ofthickness. During the growing process, SiH₄ was added to the barrierlayers or both the barrier layers and the well layers, whereconcentration of the impurities comprising silicon was rated at 1×10¹⁶to 1×10¹⁸ cm⁻³. If the growth process were suspended for a duration of 1second up to 180 seconds between the barrier layer and the well layer orbetween the well layer and the barrier layer, levelness of individuallayer is promoted and causes the width of luminescence half value to bedecreased favorably.

When adding arsenic (As) to the active layer, it is recommended to addarsine (AsH₃) or TBAs (tertiary butyl arsine). When adding phosphor tothe active layer, it is recommended to add PH₃ (phosphine) or TBP(tertiary butyl phosphine). When adding antimony (Sb) to the activelayer, it is recommended to add TMSb (trimethyl antimony) or TESb(triethyl antimony). Further, when forming the active layer, it isallowable to use, instead of NH₃, N₂H₄ (hydrazine), or C₂N₂H₈ (dimethylhydrazine), or any other organic material containing nitrogen may beused for sufficing nitrogen material.

Next, the substrate 10 was again heated up to 1050° C., so as to enablethose layers described below to be grown in the sequence of thefollowing: a p-type carrier block layer 1107 comprisingAl_(0.3)Ga_(0.7)N having 20nm of thickness, a p-type GaN composite beamguide layer 1108 having 0.1 μm of thickness, a p-type Al_(0.1)Ga_(0.9)Nclad layer 1109 having 0.5 μm of thickness, and a p-type GaN compositecontact layer 1110 having 1 μm of thickness, respectively. To sufficethe p-type impurities, EtCP₂Mg (bis-ethyl cyclopentadienyl magnesium)was used, and magnesium was added to the above layers by an amountexpressed as 1×10¹⁸ cm⁻³ to 2×10²⁰ cm⁻³. It is desired that the densityof the p-type impurities added to the p-type GaN contact layer 1110 beincremental in the direction of the positive electrode 15. Thisarrangement will cause the contact resistance caused by provision of thepositive electrode 15 to be lowered. Further, in order to removeresidual hydrogen in the p-type layers obstructing activation ofmagnesium functioning as p-type impurities, it is allowable to infuse aminimal amount of oxygen into the p-type layers on the way of growingthemselves.

After causing the p-type GaN contact layer 1110 to be grown by executingthe above serial processes, atmosphere inside of the reactor of theMOCVD deposition device was fully replaced with nitrogen carrier gas andNH₃ and then lowered inner temperature by 60° C. per minute. When thesubstrate temperature reached 800° C., supply of NH₃ gas wasdiscontinued, and then the substrate was held on at 800° C. for 5minutes before being lowered to room temperature. It is desired that theintermediate substrate temperature be held on in a range from 650° C. upto 900° C. for a minimum of 3 minutes up to a maximum of 10 minutes.Preferably, the substrate temperature be lowered at a rate of 30° C. perminute or higher.

The grown film was then evaluated by applying the Raman measuringmethod. It was confirmed that characteristics proper to conversion intothe positive polarity was evidenced (in other words, it was found thatmagnesium component was activated) immediately after completing thelayer growth even when omitting an annealing process for conversion intothe positive polarity after extracting the produced wafer from the MOCVDdeposition device. It was further confirmed that the contact resistancevalue caused by formation of the positive electrode was lowered. Bycombining the above processing steps with the conventional annealingprocess for conversion into the positive polarity, efficiency in theactivation of magnesium component was further promoted favorably.

In the crack-preventive layer 1103 comprising In_(0.07)Ga_(0.93)N, it isallowable to arrange the In composite ratio into any value other than0.07, and further, it is allowable to omit the InGaN crack preventivelayer 1103 itself. However, if the lattice incompatibility between theclad layer 1104 and the GaN composite substrate 10 becomes excessive, itis recommended that the InGaN crack preventive layer 1103 be insertedbetween them for preventing crack from occurrence. Further, in order toprevent crack from occurrence, it is preferred to use germanium in placeof silicon for constituting the n-type impurities in the aboverespective n-type layers.

The active layer 1106 are initially composed of a barrier layer andcompleted with formation of the last barrier layer. Instead of thisarrangement, it is also allowable to constitute the active layer 1106 byinitially laying a well layer and completing with the last well layer.Further, not only the above-cited 3 layers, but the number of the welllayers may be a maximum of 10 layers. This is because, provision of amaximum of 10 of the well layers lowered threshold current density andenabled continuous oscillations at room temperature. Especially, when 2layers up to 6 layers of the well layer were provided, the thresholdcurrent density was noticeably lowered as a desirable factor. Notefurther that aluminum component may also be included in the above activelayer.

Although the above-described constitution added a specific amount ofsilicon impurities to both the well layers and the barrier layersjointly constituting the active layer 1106, impurities may not always beadded thereto. However, it was proven that luminous intensity wasnoticeably promoted when impurities comprising silicon were added to theactive layer 1106. Not only silicon, but any of those elements includingoxygen, carbon, germanium, zinc, and magnesium, or combination of morethan two of them may also be used. It was found that approximately1×10¹⁷ to 8×10¹⁸ cm⁻³ in terms of the sum of added impurities wasfavorable. It should be noted that the object of adding impurities isnot always defined to be both the well layers and the barrier layers,but it is also allowable to add impurities to either of them.

The p-type carrier block layer 1107 may not always be composed ofAl_(0.3)G_(0.7)N. By adding indium to the AlGaN composite element,conversion into the positive polarity can be promoted via growth underlower temperature, thereby minimizing damage incurred to the aboveactive layer 1106 on the way of growing crystals. Although the carrierblock layer 1107 itself may be omitted, incorporating the carrier blocklayer 1107 lowered the threshold current density. This is because thecarrier block layer 1107 functions so as to confine carrier in theactive layer 1106. By lifting the Al composite ratio in the carrierblock layer 1107, practical effect of confining the carrier is enhancedfavorably. Further, by decreasing the Al composite ratio down to acritical limit just enough to confine the carrier, mobility of thecarrier inside of the carrier block layer 1107 is promoted to cause theelectrical resistance to be lowered favorably.

The above p-type clad layer 1109 and the n-type clad layer 1104 arerespectively composed of crystals comprising Al_(0.1)Ga_(0.9)N. Instead,it is also allowable to prepare tertiary crystals comprising AlGaNhaving the Al composite ratio other than 0.1. By increasing thecomposite crystalline ratio of aluminum, difference in the energy gapand the difference in the refraction index from that of the active layer1106 respectively expand, thus enabling the carrier and light to beconfined in the active layer 1106 effectively, and yet, enabling tolower the current density at the threshold value of the laseroscillation. Further, by decreasing the Al composite ratio down to acritical limit just enough to continuously confine the carrier andlight, mobility of the carrier in the clad layers 1104 and 1109 ispromoted, thereby making it possible to lower operating voltage of thesemiconductor laser element. Taking this into account, it is desirableto arrange the Al composite ratio in the p-type clad layer 1109 in arange from 0.06 to a maximum of 0.09.

Desirably, thickness of the n-type AlGaN clad layer 1104 shall bedefined in a range from 0.7 μm to a maximum of 1.5 μm. By way of formingthe suggested thickness, effect of making the vertical transverse modesingle-peaked and the effect of confining light are enhanced, thusmaking it possible to improve optical characteristics of laser and lowerthe current density of the threshold value of laser.

The above-referred clad layers 1104 and 1109 are respectively composedof tertiary composite crystals comprising AlGaN. However, the cladlayers 1104 and 1109 may also be composed of quaternary compositecrystals comprising AlInGaN, AlGaNP, AlGaNAs, or the like for example.Further, in order to lower electric resistance, the p-type clad layer1109 may consist of a super-lattice structure comprising a p-type AlGaNlayer and a p-type GaN layer, or another super-lattice structurecomprising a p-type AlGaN layer and a p-type AlGaN layer, or anothersuper-lattice structure comprising a p-type AlGaN layer and a p-typeInGaN layer.

The above description has solely referred to the crystalline growthmethod by applying the MOCVD deposition device. However, it is alsoallowable to apply either the molecular beam epitaxial (MBE) growthmethod or the hydride vapor phase epitaxy (HVPE) method as well.

Inasmuch as the steps for processing an epitaxial wafer into individualchips of the nitride composite semiconductor laser element have alreadybeen described in detail, further description thereof is omitted. Notethat, in the course of processing the epitaxial wafer, the lateral width(length in the direction perpendicular to the laser beam guide region14) W of the semiconductor laser element 3 was arranged to be 400 μm.

The above-described dislocation concentrated regions 11 are respectivelydisposed inside of an original n-type GaN composite substrate 10 acrosspitch P=400 μm. A high-luminescence region 25 is disposed at the centerof the individual regions accommodating low dislocation. Further, asdescribed earlier, by arranging the distance (d) between the laser beamguide region 14 and the dislocation concentrated region 11 to be 80 μmand the distance (t) between the laser beam guide region 14 and the highluminescence region 25 to be 120 μm, an individual semiconductor laserelement in the form of a chip includes a single line of the region 11concentrated with dislocation and a single line of the high luminescenceregion 25. Concretely, in this embodiment, the lateral width W=Pitch P,where 2(t+d)=P.

(Characteristics of the Semiconductor Laser Element)

Since the nitride composite semiconductor laser element 3 related tothis embodiment incorporates a current contracting portion at an optimalposition, the semiconductor laser element 3 proved to have attained morethan 5000 consecutive hours of laser oscillation service life under anexperimental condition at 60 mW of laser output and 70° C. in theenvironmental atmosphere. Incidentally, as a result of a comparativeexperiment based on the identical condition executed against samples ofsemiconductor laser element manufactured by applying a conventionaltechnique, the comparative samples merely withstood approximately 1000consecutive hours of own service life.

(Positional Relationship Between the Laser Beam Guide Region and theSubstrate)

As described earlier, the nitride composite semiconductor substrate 10according to this embodiment is characterized by the constitutionincorporating stripe-form regions concentrated with dislocation,discrete regions accommodating low density dislocation, highlyluminescent regions, or the center portion of the low-densitydislocation region. Further, the above described manufacturing methodalso features the present invention. In the course of manufacturing thenitride composite semiconductor laser element using the above nitridecomposite semiconductor substrate, inventors of the present inventiondetected that, depending on the local position to be determined for theformation of the laser beam guide region of the nitride compositesemiconductor laser element on the substrate, the laser oscillationservice life was variable. A preferable range of the above-referreddistances “d” and “t” is described below.

FIG. 9 presents a graphic chart, which was plotted after executing thefollowing processes described below. Initially, a certain number ofsemiconductor laser elements each having the laser beam guide regions 14at changed positions were manufactured on an n-type GaN substrate 10having P=600 μm of pitches by applying substantially the same method asperformed for the semiconductor laser element 3 of this embodiment, andthen, the laser oscillation service life was measured via plottingagainst the distance “d” between the laser beam guide region 14 and thedislocation concentrated region 11. The measured result is plotted inFIG. 9. The maximum value of the distance “d” was 200 μm. The measuredresult was evaluated based on the condition of using 60 mW of the laseroutput at 70° C. in the environmental atmosphere. In consequence, it wasconfirmed that, when the distance “d” was longer than or equal to 40 μm,3000 consecutive hours of practical service life was achieved, andfurther, when the distance “d” was longer than or equal to 60 μm, morethan 5000 consecutive hours of practical service life was secured, thusacquiring sufficiently durable characteristics. Note that themeasurement of service life was carried out for a period up to 5000consecutive hours. The upward arrowed marks shown in FIG. 9 respectivelyindicate that the laser oscillation service life is extensible beyond5000 consecutive hours.

When the distance “d” is shorter than or equal to 10 μm, the laser beamguide region 14 is generally positioned above the dislocationconcentrated region 11. In this condition, the laser oscillationcharacteristics extremely degraded own service life to be less thanseveral hours. When the above distance “d” ranges from 20 μm to 30 μm,it was noted that the drive current gradually rose relative to theelapse of time, and then, when the drive current reached approximately200 mA, due to thermal runaway, laser oscillation became no longerpossible. Further, as a result of evaluating leak current from the p-njunction by adding inverse bias current to the p-n junction, compared tosuch a laser element having the distance “d” between the laser beamguide region 14 and the dislocation concentrated region 11 being longerthan or equal to 60 μm, it was confirmed that the laser element causingthe drive current to gradually rise apparently generated substantialleak current which was incremental relative to the elapse of time.

In the case of such a semiconductor laser element formed in the vicinityof the dislocation concentrated region 11 of the GaN composite substrate10 prepared via the above processes, because dislocation (defect) hasbeen generated at a p-n junction close to the laser beam guide region14, or because the p-n junction surface remains irregular withoutlevelness caused by generation of the above defect, leak current whichis incremental relative to the elapse of the driving time has beengenerated, thereby presumably causing the life characteristics to havebeen degraded. In consequence, it was found to be necessary to set thedistance “d” at least to be longer than or equal to 40 μm, preferably tobe longer than or equal to 60 μm.

FIG. 10 presents a graphic chart, which was plotted after executing thefollowing processes. Initially, a certain number of semiconductor laserelements each having the laser beam guide regions 14 at changedpositions were manufactured on an n-type GaN composite substrate 10having P=600 μm of pitches by applying substantially the same method asperformed for the semiconductor laser element 3 of this embodiment, andthen, the laser oscillation service life was measured via plottingagainst the distance “t” between the laser beam guide region 14 and thehigh luminescence region 25. The maximum value of the distance “t” was200 μm. The measured result was evaluated based on the condition ofusing 60 mW of the laser output at 70° C. in the environmentalatmosphere. As a result, it was confirmed that, when the distance “t”was longer than or equal to 30 μm, more than 3000 consecutive hours ofpractical service life was achieved, and further, when the distance “t”was longer than or equal to 50 μm, more than 5000 consecutive hours ofpractical service life was secured, thus acquiring sufficiently durablecharacteristics. Note that, like the one shown in FIG. 9, the upwardarrowed marks shown in FIG. 10 respectively indicate that the laseroscillation service life is extensible beyond 5000 consecutive hours.

When the distance “t” is 0 μm, the laser beam guide region 14 ispositioned above the high-luminescence region 25. In this case, it wasdetected that characteristics were degraded down to approximately 100hours of service life. When the laser beam guide region 14 was disposedin the vicinity of the high-luminescence region 25, voltage of thesemiconductor laser element and the drive current value rose, and thus,it was conceived that the rise of the voltage of the laser element andthe drive current value caused the service life of the laser element tobe degraded. There are some regions containing higher resistance valuesthan that of peripheral portions in the high-luminescence region 25 (orin the center portion of low-dislocation region) and nearby portionswithin the substrate 10. It is thus conceived that the presence of thehigher resistance regions adversely affected the voltage of the laserelement. In the close observation of the voltage of the laser element,voltage is substantially constant when the distance “t” is longer thanor equal to 50 μm. However, the voltage slightly rises in such a regionwhere the distance “t” is less than 50 μm. As a result, it was concludedthat at least “t”≧30 μm, preferably “t”≧50 μm should essentially beprovided.

(Preferable Range of the Substrate)

Next, preferable range of pitch P capable of causing the spheres 11concentrated with dislocation to appear in the GaN composite substrate10 was studied. As described earlier, it is quite essential that, in thecourse of manufacturing the GaN composite substrate 10, while exposingthe facet 41-24 sectional form of the surface be provided withsaw-teethed projections and recesses. As a result of this arrangement,it is possible to generate the low-dislocation regions and thehigh-luminescence regions 25. A specific portion remote from both thedislocation concentrated regions 11 and the high-luminescence regions 25by a required distance corresponds to a proper portion suitable forforming the laser beam guide region. Accordingly, there will be noavailable portion for forming the laser beam guide region unless morethan double the sum 70 μm comprising the minimum value 40 μm as thedistance required for generating the above effect and the minimum value30 μm as the distance “t” for generating the above effect can be securedas the pitch P. Due to this reason, range of the distance P is definedas P≧140 μm.

From the viewpoint whether the growth can be sustained while holding thepositions of the projections and recesses constant or not in theformation of the GaN composite substrate 10, the value of the distance Pis restricted. This is because, if the actual positions of theprojections and recesses were displaced from the initial positionsrelative to the progress of the growth, the pitch P becomes inconstant,thereby making it difficult to set the laser beam guide region at aspecific position against the produced GaN composite substrate 10. Afterstudying the practical range of the pitch P by taking the above reasonsinto account, it was understood that the above condition could hardly besustained when the pitch P was less than 50 μm, and thus, it was foundthat the pitch P should desirably be more than or equal to 100 μm.

Further, from the viewpoint of the function for causing the dislocationto be concentrated into the regions 11, it would be better to cause thefacet {11-22} to be generated massively. When the pitch P is arranged tobe more than or equal to 300 μm, as described above, defect density inthe dislocation concentrated region 11 and the defect density in the lowdislocation region favorably differ from each other by more than threeorders of magnitude. Although there is no restriction on the upper limitof the distance P, if the distance P becomes excessive, projections andrecesses of the produced ingot also become excessive, thus resulting inthe decreased number of procurable wafers. Accordingly, approximately1000 μm would be appropriate for the upper limit of the distance P. Inconclusion, it is defined that 140 μm≦P is essential, preferably 300≦Pis ideal. If it is required to define the upper limit, P≦1000 isappropriate for use.

Fourth Embodiment of the Present Invention

Although constitution of the semiconductor laser element related to thefourth embodiment is identical to that of the semiconductor laserelement 3 of the third embodiment, values of the distance P, d, and t,differ from each other. FIG. 11 schematically illustrates dispositionrelationship between the laser beam guide region 14 formed in thesemiconductor laser element, the dislocation concentrated region 11 andthe high-luminescence region 25 present in the n-type GaN compositesubstrate 10. Local regions for constituting individual semiconductorlaser elements (chips) in the n-type GaN composite substrate 60 aredesignated by the reference numerals 61 and 62. Note that, in thefourth, fifth, and the sixth embodiments, positional relationshipbetween the laser beam guide region 14, the dislocation concentratedregion 11, and the high-luminescence region 25, is different from thatof the third embodiment. However, like the one shown in the thirdembodiment, the semiconductor laser element is designated by thereference numeral 3 in the fourth, fifth, and the sixth embodiments.

When executing the method of manufacturing the semiconductor laserelement 3 according to the fourth embodiment, the pitch P betweenindividual regions 11 concentrated with dislocation in the n-type GaNcomposite substrate 60 is defined as 500 μm. Two units of laser beamguide regions 14 are disposed per pitch, thereby forming two units ofsemiconductor laser elements (chips) 3 per pitch. Each of the twosemiconductor laser elements 3 (shown as the regions 61 and 62) isprovided with 250 μm of lateral width.

In the region 61, the distance “d” between the laser beam guide region14 and the closest region 25 concentrated with dislocation is defined as100 μm, whereas the distance “t” between the laser beam guide region 14and the closest region 25 accommodating low-density dislocation isdefined as 150 μm. In the other region 62, the distance “d” between thelaser beam guide region 14 and the closest region 11 concentrated withdislocation is defined as 150 μm, whereas the distance “t” between thelaser beam guide region 14 and the closest center portion 25 between thelow-density dislocation accommodating regions is defined as 100 μm.

There is a specific relationship designated as 2W=P between the width Wof the above-referred regions 61 and 62 and the pitch P of the region 11concentrated with dislocation, where 2(t+d)=P. When expressing thedistance “d” and “t” in the region 61 in terms of “d1” and “t1” and thedistance “d” and “t” in the other region 62 in terms of “d2” and “t2”,these are in the relationship expressed as “dl”=“t2” and “d2”=“t1”.

In the region 61, only one unit of the dislocation concentrated region11 is present inside of the n-type GaN composite substrate 60 withoutpresence of the high-luminescence region 25. In the other region 62,only one unit of the high-luminescence region 25 is present in then-type GaN composite substrate 60 without presence of the region 11concentrated with dislocation. Also in the present fourth embodiment,range of the distance d, t, and P effective for the generation of thepractical effect of the present invention conforms to the description onthe preceding third embodiment. Those practical effects described in thethird embodiment can be secured within the range of the distance d, t,and P even when implementing the fourth embodiment.

Fifth Embodiment of the Present Invention

In the method of manufacturing the semiconductor laser element accordingto the fifth embodiment, the number of semiconductor laser element(chip) 1 to be produced per pitch P is increased to four. Like the oneshown in FIG. 11, FIG. 12 schematically illustrates the dispositionrelationship between the laser beam guide region 14 formed in thesemiconductor laser element, the dislocation concentrated region 11present in the n-type GaN composite substrate, and the high-luminescenceregion 25 formed in this substrate. Those regions corresponding toindividual semiconductor laser elements (chips) in the n-type GaNcomposite substrate 70 are respectively designated by the referencenumerals 71 to 74.

In the method of manufacturing the semiconductor laser element 3according to the fifth embodiment, the pitch P corresponding to thedislocation concentrated region 11 formed in the n-type GaN compositesubstrate 70 is defined as 800 μm, in which four of the laser beam guideregions 14 are disposed per pitch so as to form four of thesemiconductor laser beam guide regions (chips) per pitch. Each of thesemiconductor laser elements 3 (corresponding to the regions 71 to 74shown in FIG. 12) has an even lateral width W, which is defined as 200μm. In each of the serially aligned regions 71, 72, 73, and 74, thedistance “d” between the laser beam guide region 14 and the closestregion 11 concentrated with dislocation is defined as 80 μm, 28 μm, 320μm, and 120 μm, respectively. The distance “t” between the laser beamguide region 14 and the closest center portion 25 of the low densitydislocation accommodating region in respective regions 71 to 74 isdefined as 320 μm, 120 μm, 80 μm, and 280 μm, respectively.

As shown in FIG. 12, the region 71 includes a unit of the region 11concentrated with dislocation, the region 73 includes a unit ofhigh-luminescence region 25, whereas neither of the regions 72 and 74include the region 11 concentrated with dislocation or thehigh-luminescence region 25. According to the method of manufacturingthe semiconductor laser element 3 based on the fifth embodiment, thereis a relationship expressed as 4W=P, and further, there is arelationship expressed as 2(t+d)=P against each of the laser beam guideregions 14. Those practical effects described in the third embodimentcan also be secured within the range of the distance d, t, and P, evenwhen implementing the fifth embodiment.

Sixth Embodiment of the Present Invention

In contrast with the fourth or fifth embodiment, the sixth embodimentintroduces the relationship expressed as width W>pitch P. Like the oneshown in FIG. 11, FIG. 13 schematically illustrates the dispositionrelationship between the laser beam guide region 14 formed in thesemiconductor laser element, the region 11 concentrated with dislocationpresent in the n-type GaN composite substrate, and the high-luminescenceregion 25. The region to be constituted into the semiconductor laserelement (chip) 3 within the n-type GaN composite substrate 80 isdesignated by a reference numeral 81.

According to the method of manufacturing the semiconductor laser elementbased on the sixth embodiment, the pitch P of the region 11 concentratedwith dislocation present in the n-type GaN composite substrate 80 isdefined as 200 μm. The lateral width W of the semiconductor laserelement 3 (corresponding to the region 81 shown in FIG. 13) is definedas 300 μm. Concretely, it is arranged to form two third (⅔) unit ofsemiconductor laser element (chip) 3 per pitch (in other words, formsone chip 3 per 1.5 pitch). The distance “d” between the laser beam guideregion 14 and the closest region 11 concentrated with dislocation isdefined as 50 μm, whereas the distance “t” between the laser beam guideregion 14 and the closest center portion 25 of the low-densitydislocation accommodating region is defined as 50 μm.

As shown in FIG. 13, at least one unit or more than one unit of thedislocation concentrated region 11 and the high-luminescence region 25is contained in a unit of semiconductor laser element (chip). In themethod of manufacturing the above semiconductor laser element accordingto the sixth embodiment, (⅔)W=P, and further, 2(t+d)=P against each ofthe laser beam guide regions 14.

Those practical effect described for implementing the third embodimentcan also be secured within the distance range d, t, and P, even whenimplementing the sixth embodiment.

In the above third to sixth embodiments, the description has seriallyreferred to the arrangement for forming one unit, two unit, four unit,and two third (⅔) unit of semiconductor laser element per pitch.However, applicable range of the present invention is not solely limitedto the above range, but it is also possible to dispose 6 units, one half(½) unit, and one third (⅓) unit as well. Further, by way of arrangingthat part of the semiconductor laser elements to be disposed on the GaNcomposite substrate 10 may exceed a predetermined range of the distance“t” and “d”, it is also possible to provide the above laser elements bya certain number other than the value that can be expressed in terms ofmultiple integral number or simple fraction number.

Seventh Embodiment of the Present Invention

The seventh embodiment replaces the nitride semiconductor laser element3 incorporating the ridge-stripe form configuration described in thepreceding third to sixth embodiments with a nitride compositesemiconductor laser element 4 incorporating a current inhibiting layer.Referring now to FIG. 14, a constitution of the nitride compositesemiconductor laser element 4 incorporating a current inhibiting layeris described below.

Concretely, the semiconductor laser element 4 according to the seventhembodiment of the present invention comprises the following: an n-typeGaN composite substrate 10, and those serially laminated layers disposedon the substrate 10 including; an n-type GaN layer 1201, an n-typeIn_(0.07)Ga_(0.93)N crack preventive layer 1203, an n-typeAl_(0.1)Ga_(0.9)N clad layer 1204, an n-type GaN beam guide layer 1205,an active layer 1206, a p-type Al_(0.2)Ga_(0.8)N carrier block layer1207, a p-type GaN beam guide layer 1208, a p-type Al_(0.1)Ga_(0.9)Nfirst clad layer 1209 a, a current inhibiting layer 1220, a p-typeAl_(0.1)Ga_(0.9)N second clad layer 1209 b, a p-type InGaN contact layer1210, a positive electrode 15, and a negative electrode 16.

The current inhibiting layer 1220 may consist of a layer capable ofinhibiting current so as to enable current flowed from the positiveelectrode 15 to solely permeate through width-wise interval of thecurrent inhibiting layer 1220. For example, the current inhibiting layer1220 may be composed of an n-type Al_(0.25)Ga_(0.75)N layer. The Alcomposite ratio of the current inhibiting layer 1220 is not alwayslimited to be 0.25, but any other value may be applied. In the presentembodiment, an opening of the current inhibiting layer 1220 behaves incorrespondence with the laser beam guide region 14. Practical effects ofthe seventh embodiment described in the third embodiment can also besecured within the range of the distance d, t, and P, even whenimplementing the present embodiment.

Eighth Embodiment of the Present Invention

The eighth embodiment arranges that at least any of the elementsincluding arsenic (As), phosphor (P), and antimony (Sb), is included inthe active layer of the above nitride composite semiconductor laserelement 3 or 4. Except for this arrangement, constitution of the nitridecomposite semiconductor laser element 3 or 4 remains as was alreadydescribed in the preceding embodiments.

In the eighth embodiment, it is so arranged that at least any of thethree elements including As, P, and Sb, is included at least in the welllayer among the active layer 1106 and 1206 that constitute the nitridecomposite semiconductor laser element 3 or 4. Assuming that thecomposite ratio of the sum of the above elements is X, whereas thecomposite ratio of nitrogen element included in the well layer is Y, thevalue of X is less than that of Y. Further, X/(X+Y) corresponds to avalue less than or equal to 0.3 (30%), preferably less than or equal to0.2 (20%). Further, the lower limit value of the sum of the above threeelements is greater than 1×10¹⁸cm⁻³.

When the composite ratio rose higher than 20%, concentration separation,i.e., a phenomenon in which the composition of the above-mentionedelements varies from one region to another within the quantum welllayer, starts to occur. When the composite ratio X rose further than30%, it causes the depth separation to shift itself to crystallineseparation intermingled with hexagonal crystals and cubic crystals,thereby causing the crystalline characteristics of the well layer to belowered. On the other hand, when the added sum of the above elementsbecomes less than 1×10¹⁸ cm⁻³, practical effect of adding the aboveelements to the well layer can no longer be secured.

Practical effect of the eighth embodiment is attributable to a smalleffective mass of electrons and holes of the well layer and asubstantial mobility of electrons and holes of the well layer. In thecase of the semiconductor laser element, the former advantage means thatdistribution of carrier inversion required for generating laseroscillation by feeding a small amount of current. The latter advantagemeans that, even if electrons and holes are extinguished in the activelayer by effect of the reunion of luminescence, due to diffusion,electrons and holes can be infused anew at an extremely fast rate. Inother words, compared to such a conventional nitride semiconductor laserelement totally devoid of any of the above elements including As, P, andSb, in the conventional active layer thus far being reported, thenitride semiconductor laser element according to the eighth embodimentof the present invention features low density of current at thresholdvalue and excellent characteristics in the noise suppression. Practicaleffect of the present invention as described in the third embodiment canalso be secured within the range of the distance d, t, and P, even whenimplementing the eighth embodiment.

Ninth Embodiment of the Present Invention

When forming individual nitride semiconductor laser elements on asubstrate, the ninth embodiment applies a selective growth technique.Except for this application, all the aspects are identical to any of thepreceding third to eighth embodiments.

The selective growth technique consists of those serial processesdescribed below. An opening provided mask composed of growth inhibitingmaterial including oxide such as silicon dioxide and nitride such as SiNor AlN is previously formed on a substrate. When forming individualnitride semiconductor layers on the substrate, the selective growthtechnique controls growth processes in order that the growth can bepromoted in the lateral direction at the initial stage of the growth. Byapplying this technique, it is possible to effectively prevent crackfrom potentially being generated relative to the growth of individualnitride semiconductor layers. The mask can be formed in correspondencewith the surfaces right above the dislocation concentrated region 11 andthe high-luminescence region 25, or the mask may also be formedindependently of the location cited above. However, in order toeffectively prevent crack from potentially being generated in the laserbeam guide region, it is preferred to form the mask right beneath thelaser beam guide region 14.

Tenth Embodiment of the Present Invention

Referring now to FIG. 15, the tenth embodiment complete with practicalapplication of the above-described nitride semiconductor laser elementto a semiconductor optical device such as an optical pickup system isdescribed below. FIG. 15 is a simplified schematic block diagram forexplanatory of the inner constitution of a semiconductor optical deviceaccording to the tenth embodiment of the present invention. Thefollowing description refers to an example of applying theabove-described nitride semiconductor laser element to an optical discdrive unit.

The optical disc drive unit shown in FIG. 15 consists of the following:a spindle motor 702 for rotating an optical disc 701 in thecircumferential direction; an optical pickup unit 703 for readinginformation by causing the optical disc 701 to be irradiated by laserbeam; and a controlling circuit 704 for controlling operations of thewhole components of the optical disc drive unit. It should be noted thatthe optical pickup unit 703 shifts itself in the diametrical directionof the optical disc 701 via function of an actuator (not shown) drivenby the controlling circuit 704.

In the above optical disc drive unit shown in FIG. 15, the opticalpickup unit 703 consists of the following: a laser unit 705incorporating a nitride semiconductor laser element for emitting laserbeam; a beam splitter 706 which enables laser beam emitted from thelaser unit 705 to permeate through the beam splitter itself and guideslaser beam from a follow-up mirror 708 to a beam detector 707; a beamdetector 707 which detects laser beam from the beam splitter 706 andthen transmits the beam-detect signal to a controlling circuit 704; afollow-up mirror 708 which guides laser beam from the laser unit 705 toan optical disc 701 and then guides laser beam reflected from theoptical disc 701 to the beam splitter 706; and an object lens 709 whichcauses laser beam reflected from the follow-up mirror 708 to becondensed on the optical disc 701.

As described above, the inventive nitride composite semiconductor laserelement installed in the laser unit 705 is capable of generating 330 to550 nm of oscillating wavelength and stably operating itself at 30 mW ofhigh output condition at 60° C. of high temperature in the environmentalatmosphere, and yet, further features its durable laser oscillatingservice life. Accordingly, since the shorter the oscillatingwave-length, the higher the recording and reproducing density to berealized, the inventive nitride composite semiconductor laser element ismost suitable for application to an optical disc drive unit designed forthe recording and reproduction of high-density optical data with highreliability.

In the course of recording information by operating the optical discdrive unit featuring the above constitution, laser beam output from thelaser unit 705 is modulated in correspondence with input data from thecontrolling circuit 704, and then, the modulated laser beam passesthrough the beam splitter 706 and then reflected by the follow-up mirror708 before eventually being irradiated onto the optical disc 701 via theobject lens 709, thus enabling information to be recorded on the opticaldisc 701. Alternatively, magnetic field transferred onto the recordingsurface of the optical disc 701 is modulated in response to theinformation received from the controlling circuit 704 before eventuallycausing the modulated information to be recorded on the optical disc701.

In the course of reproducing information, laser beam is subject tooptical variation by the pit array on the optical disc 701, and thenpasses through the object lens 709. The laser beam is then reflected bythe follow-up mirror 708, and then reflected by the beam splitter 706before eventually being detected by the beam detector 707 so as togenerate a reproduction signal. These serial processes are regulated bythe controlling circuit 704. The above semiconductor laser elementoutputs laser beam containing 30 mW of the recording power andapproximately 5 mW of the reproducing power for example.

Further to the above-referred optical disc drive unit incorporating theabove optical pickup system, the nitride composite semiconductor laserelement according to the present invention is also applicable to a laserprinter, bar-code reader, a projector using laser beam comprising threeprimary colors including blue, green, and red, or the like.

INDUSTRIAL APPLICABILITY

The present invention is suitable for constituting a laser beamgenerating source featuring high-output capacity and durable servicelife and also for constituting the method for forming the laser beamgenerating source. In addition, the prevent invention is effectivelyapplicable to constitute lighting source of a variety of optical devicessuch as an optical disc drive unit, laser printer, or a bar-code reader,or the like, as well.

1. A nitride semiconductor laser device comprising: a nitridesemiconductor substrate having a stripe-shaped dislocation-concentratedregion in which a crystal defect concentrates and a low-dislocationregion elsewhere than in the dislocation-concentrated region; a nitridesemiconductor layer formed on the nitride semiconductor substrate; and agrowth-inhibiting film formed on a surface of the nitride semiconductorsubstrate, in a position covering the dislocation-concentrated region,for inhibiting growth of a nitride semiconductor crystal, wherein thenitride semiconductor layer is formed by forming the nitridesemiconductor crystal on part of the nitride semiconductor substratewhere the growth-inhibiting film is formed.
 2. The nitride semiconductorlaser device according to claim 1, wherein the growth-inhibiting filmhas a linear shape, and for each dislocation-concentrated region likesaid dislocation-concentrated region, there are provided, as saidgrowth-inhibiting film, a plurality of growth-inhibiting films in ashape of slats, so that said each dislocation-concentrated region iscovered by said plurality of growth-inhibiting films
 3. The nitridesemiconductor laser device according to claim 2, wherein said pluralityof growth-inhibiting films provided for said eachdislocation-concentrated region have a width of 1 μm or more but 10 μmor less each, and are arranged parallel to one another with an intervalof 1 μm or more but 10 μm or less between one growth-inhibiting film andthe next, so that a region encompassing widths of and intervals betweensaid plurality of growth-inhibiting films covers said eachdislocation-concentrated region.
 4. The nitride semiconductor laserdevice according to claim 1, further comprising: a GaN film formed onthe surface of the nitride semiconductor substrate so as to cover thegrowth-inhibiting film, the GaN film having n-type conductivity, whereinthe nitride semiconductor substrate has n-type conductivity.
 5. Thenitride semiconductor laser device according to claim 4, wherein the GaNfilm having n-type conductivity has a thickness of 1 μm or more but 20μm or less.
 6. The nitride semiconductor laser device according to claim1, wherein the growth-inhibiting film has a thickness of 0.05 μm or morebut 1 μm or less.
 7. The nitride semiconductor laser device according toclaim 1, wherein the growth-inhibiting film is a silicon compound filmor a metal film.
 8. The nitride semiconductor laser device according toclaim 7, wherein the growth-inhibiting film is one of a SiO₂ film, aSi₃N₄ film, a titanium film, and a tungsten film.
 9. The nitridesemiconductor laser device according to claim 1, wherein the nitridesemiconductor layer has a quantum well active layer, and the activelayer includes a well layer formed of In_(x)Ga_(1-x)N (where 0<x<1). 10.The nitride semiconductor laser device according to claim 9, wherein theactive layer contains at least one element selected from the group ofAs, P, and Sb.
 11. The nitride semiconductor laser device according toclaim 1, wherein the nitride semiconductor substrate is a GaN substrate.12. A semiconductor optical device comprising the nitride semiconductorlaser device according to claim 1 as a light source.
 13. The nitridesemiconductor laser device according to claim 1, wherein thedislocation-concentrated region has a width of 10 μm or more but 40 μmor less.
 14. The nitride semiconductor laser device according to claim1, wherein the dislocation-concentrated region is polycrystalline. 15.The nitride semiconductor laser device according to claim 1, wherein thedislocation-concentrated region is slightly inclined relative to thelow-dislocation region.
 16. The nitride semiconductor laser deviceaccording to claim 1, wherein the nitride semiconductor substrate isformed solely of a nitride semiconductor.